Yes, both parks have a phenomenal amount of chairs and you'll have no trouble finding one...or a group of them. Many are under shade, that's not a problem either. You'll find them quite easily. If you're bringing valuables in, you may want to rent a locker to store them (they're toward the front of the park). Otherwise, if it's stuff you don't care all that much about...i.e. my t-shirt or DW's coverup for her bathing suit, our lunch (we pack our own food), towels, sunscreen, we just leave it on our chairs and go. I've heard of people having stuff stolen, and I'm sure it does happen. But over the past 13 years, we've made probably 50 separate visits to the water parks and nobody has ever touched our stuff. And as I said, we don't leave anything of value anyway, so if they really want it, they're welcome to it. To be specific, for us we like the following spots to grab chairs.... In TL, there's a beach area that's near Miss Adventure Falls and Crush n Gusher. That area has plenty of shade in spots, including several large covered huts. We grab chairs there. In BB, we go to the area right by Ski Patrol (across from the bath house that's there), and get chairs. But that's just one of several places you can go. Finding a spot won't be an issue.
As for strategy, it all depends on what you want to do. Generally speaking, we hit the slides first because they get lines. Then when we're done with those, which is when the crowds arrive, we hit the wave pool or lazy river, since they don't get lines. Your 4 year old is going to be too young for the biggest slides in the parks, but there are still fun ones he/she can do. At TL, maybe head for Crush n Gusher (if he/she is adventurous) or Miss Adventure Falls. At BBB, maybe spend time in Ski Patrol (it doesn't get long lines really, but it can get a bit crowded). The wave pools at the two parks are vastly different. TL has massive waves that come once every 90 seconds to two minutes. So only strong swimmers should go in above their knees. However, since it's a zero entry pool, kids can walk in up to ankle depth and still really enjoy it (by time the waves get that far back, they're not that strong). We've been bringing our kids into it since they were 1, and they LOVE it. At BBB, the waves are much smaller and are constant. Sort of like being out on a choppy ocean. Again, it's a zero entry pool, so you can go as shallow as you want. Both have really fun lazy rivers.
If you have the ability to do big slides on your own (trade off with spouse, whomever), then both parks have awesome ones. Get to them early, they get crowded. Humunga Kowabunga at TL and Summit Plummet or Slush Gusher at BB. Crush n Gusher at TL is sorta/kinda like a "roller coaster" water slide in that you sit in a tube and jets of water will push you up hill and over a small crest. It's not truly a roller coaster, but I guess as close to one as a tube water slide can get. Plenty safe for a 4 year old because they have two person tubes and you can sit behind him/her and hold on. Tons of little kids ride it.