I have stayed at PPH and DLH (the latter just last week). They edge each other out in various categories, it's really hard for me to decide which one I like more! And we have a newly turned 5 year old boy...
Many of the things that draw us to DLH are things anyone can do. The grounds are walkable by anyong. The remote controlled boats, which are AWESOME (my 4-turned-5, during the trip, year old could have spent hours there), are available for anyone with adequate coinage. You can get one timed turn steering a boat for $2, you can get 4 turns for $5, or 10 turns for $10. In the future? We'll be bringing a $10 or 2, just to make sure *everyone* gets adequate time with the boats.
The pool was great fun, BUT...the big huge draw for a little kid is the big pirate ship in the middle of the pool area, and that ship is off limits, no touching, no climbing, no playing. I didn't totally understand that so didn't totally explain it to DS, and he was mighty disappointed when I had to read the signs to him. There's also a big slide near the skull rock and mountains, but you have to be a solid swimmer, you can't wear floatation devices, and you can't have anyone waiting at the bottom of the slide for you...not many 4 year olds are going to meet that description. There is a very short, simple slide over in a corner, and DS loved that, once he got over not being able to play on the ship or go down the big slide.
There's also a "quiet" pool that would definitely be a destination of our choice, should we stay there again, but it looked closed when I finally noticed it so I didn't try to go...it was not closed, I just thought it was. I guess it has a sandy area, and that would have been great fun for the kiddo.
The beds at the DLH are on big solid bedframes that go up where you don't expect and can hurt anyone sliding off in the wrong spot. They were tall for me (5'3") and I bruised myself more than once, when I forgot about the frame and slid off the foot end.
The lobby didn't excite E at all, except for the pix of Walt Disney, who he called..."the man who smoked cigarettes and died b/c he hurt his lungs", a turn of phrase he got from me, when I was explaining who he was and why he wasn't around anymore (his beloved grandfather died when DS was 2.5, my mom died before I even met hubby, and our cat died shortly after granddad, so talking about death is not uncommon around here)...he tends to remember JUST what you don't want him to say, and he says it.
We like PPH's rooms better, even though our pool-view room at DLH had a terrific view. We've gotten a DCA-view, and on our second PPH stay we were surprisingly upgraded to a suite that had a half-pool and half-parking lot view. The rooms feel bigger and brighter, and their beds haven't yet injured me.
Their pool is very basic, nothing fancy, but it's filled with water and that's almost all that DS cares about. Their slide is available to, well, pretty much all...it does NOT go into the pool, rather it ends in a relatively shallow area totally separate from the pool, so I imagine it's actually safer. We haven't gone on it though, as it was relatively chilly when we swam there, and in the late afternoon, and no one wanted to walk over there in the chilly air.
I love the feel of the PPH lobby, especially at Yuletime with the big tree. But even without the tree (all the hotels have big trees then, by the way) it's cute, with surfing Goofy standing there...
The walk from PPH to the parks involves going out the FRONT door, crossing the street, going in the GCH, walking through part of that hotel, and coming out into the last half/third of DownTown Disney. If you follow the signs inside the hotel, you'll be going out the side and walking alllll the way through DTD. Now if you want to end up in Tomorrowland, you can do that then hop on the monorail, but that only makes sense if Tomorrowland and further back is where you want to be.
The walk from the DLH is, IMO, the longest, especially if you are in a tower further away than the closer ones. We were lucky to be in the Dreams tower, which sits perpendicular to the DTD path, and that was just about the quickest walk a person can have from the DLH. Some times we took the monorail in (generally in the afternoon), but most of the time we just walked allll the way through DTD. When we wanted to start in DCA, we went through most of DTD, turned into GCH at the big gates to the right just *before* Sephora (across the walkway from Haagen Dasz), then through the lobby and out to the GCH/DCA entrance. Disney hotel guests can use that any time, and people who are eating at the GCH seem to be able to use it any time, but after the "rush" time in the morning, we've never had a problem using it without showing a key or receipt... That entrance puts you out just across from Grizzly River Run.
GCH is slowly wearing me down, but in general I don't like the "lodge" feel unless I'm actually staying at, say, Mt Rainier's lodge. Until recently I'd only seen it at Yuletime...they have an enormous tree then, and it makes the lobby REALLY DARK, and I have issues in low light, so I've been absolutely miserable (b/c I feel blind) in that lobby, until recently when I've seen it without the tree. Without the tree there's more light and it feels more airy and fun...with the tree I've felt it's dark and oppressive. Just my opinion, no one else's.

I've heard that the rooms are smaller than at the DLH and PPH. The pool looks, from outside the gates, absolutely gorgeous...if you're an adult. It hasn't yet appealed to DS, well, except that he'd swim in a mudpuddle if it was deep enough...body of water = fun to him, but paying for the surroundings at the GCH pool wouldn't impress him, though it's lovely to me.
If waterplay is big on your 4 year old's agenda, and if you're willing to look outside the Disney hotels, I would recommend the HoJo. It's totally walkable, and in fact it's probably closer, in time and actual walking distance, than PPH or DLH, and maybe even the DTD entrance to GCH (to get into the GCH you either go through DCA, or you go through DTD, and the MAIN entrance is on the complete other side so you'd never walk from the parks into that entrance). They have a pool in their "garden" area, and they have a toddler pool, hot tub, and big interactive waterplay *pirate ship*. Water sprays and dumps on the kids, and while we have only seen it, it looks like a lot of fun.
If you're at all interested in hojo, they have a disboard member discount of 15%, or a rate for AP holders of 20% off rack rate. You can also check their discussion boards at hojoanaheim.com, to check for when they have the spectacular Entertainment Book discount. This discount, when they have it (and they add dates often), is a much bigger discount from the normal hotel Entertainment Book discount, and you can really only get it by calling the Anaheim hotel *directly*, during Pacific time business hours, and asking for "in house reservations". You can use the card from any Entertainment Book, even the ones sold in New Zealand, for that discount (but you don't have to have the card from the book until you check in). You can get a basic room for as low as $59 plus taxes, so you can see it's a super bargain. HoJO is one of the only closeby offsite *hotels* with interior corridors. The rest are *mainly* motels, with outside corridors.