You could easily see both HP lands in one day. I have visited both lands in one day many times,
if that is what you want to do. Just understand HP is two lands, not one. Hogsmeade is located on Islands of Adventure and Diagon Alley is located in the park called Universal Studios. Universal charges a premium to visit both lands in one day.
So cost wise, you'll pay far less if you pick either WDW or Universal.
Second, there is more to Universal than the Harry Potter areas. I think, what folks a re saying is that you won't be able to see EVERYTHING in just 4 days.
Just talking food: WDW and Universal each have over 100 restaurants, and many are elaborately themed. Even if you ate 5 meals a day, that would only =20 meals.
Four days will = choices of one kind or another.
If you are into atmosphere and exploring, then I suggest Epcot, Magic Kingdom, and if you want to pay the extra cost of admission: Diagon Alley. I would also suggest - here's a crazy idea - consider a non-park day! We have bee to WDW many times, and stayed at just about all of the WDW. weather allowing, we usually spend half a day just at our WDW hotel. All the WDW hotels have beautiful themed pools and grounds to explore.
You cold likely spend a full 4 days just exploring all of WDW. Some of the WDW hotels are connected by (free) monorail . others are linked by (free) shuttle boats, all offer free busing to all of the parks. Animal Kingdom Lodge has animals you can see for free. You can even watch them while you dine at Sanaa. Over at the Boardwalk, you can rent a surrey bike (holds 4 people together). All the resorts have multiple gift shops to explore. Most WDW hotels offer lake views and walking paths around those lakes. Multiple WDW hotels feature live music part of the day. Anyone can sit in the lobby of the Grand Floridian for free while the live pianist plays Disney songs.
The Universal hotels are also elaborately themed, with free shuttle boats, paths, elaborate pools, and live musicians. the Hard Rock has live poolside musician every afternoon (weather permitting).
I think all the WDW and Universal hotel pools feature pool games every afternoon. Most WDW hotels also regularly offer PM movies on big outdoor screens, and other activities.
And if that isn't enough....you could also go on a Hidden Mickey scavenger hunt! Oh, do read up about hidden Mickeys. It is great fun to search for them as you explore WDW.
If that isn't enough, WDW and Universal also have themed mini golf, live dinner shows, and many shops to explore. Disney Springs has tons of shops to explore; many are highly themed. The
Lego store, for example, is decorated with a number of VERY large Lego creations. There's also a Coca-Cola store, and the World of Disney, House of Blues (live blues while you eat), and multiple AMAZING confectionary shops! The shops sell Disney themed everything: high end purses, adult cosplay, art prints, kids costumes, toys galore, Disney candies, Disney socks, Disney themed cookware, TONS of jewelry...you name it, they probably sell a Disney themed version of it! I would say Disney Springs is free, but odds are high you'll end up spending money there.
Traditionally, September was a slow month at WDW. That's why they are opening the new ride then -so it isn't slow this year. There are multiple reasons why it is a slow month: hot, hurricane season, start of the school year, frequent afternoon rain storms. I don't think it will be slow this year. If you want a slower time, go in August (before 8/29).
Also know, at WDW, dining reservations start 180 days (half a year!) before you arrive. You don't have to book any table service dining, but the best ones get fully booked that far in advance! Yes, it is that crazy! You also get to book 3 fastpasses at 60 days (if you stay at a WDW hotel). So please read about how both of those work.
Universal is a little more flexible. If you stay at the Hard Rock, Portofino, or Royal Pacific, those hotels include a perk called Express Pass. Express Pass = you don't generally wait more than about 15 for most rides. Universal is also more flexible when it comes to dining. They take reservations, but most also take walk-ups (with a wait).
Oh, one more thing...at both Universal DO understand the cost of food! Character meals and buffets can be quite $$$, but you can look at menus online. Some meals easily run $50 per person!