You have a lot of options with those batteries. Shooting for an amp draw of 10-15% of your pack gives respectable flight times, so for your 4000 mah packs that would be 40-60 amps, using between 500-700 watts (allowing for efficiency losses). Given the rule of thumb of 100 watts per pound for general sport flying and 125-150 watts for aerobatics, you'd be looking at a plane as heavy as 7 pounds (a slow floater) or as light as 3.5 pounds (high performance). So pick a plane in that range that you like and then work out what motor you need. There isn't a general Kv rule for planes. That decision is based on what prop the plane needs at what rpm. I wouldn't worry much about going to balsa if there's a balsa plane you like. Yes, they incur more damage from major crashes, but most crashes are just nicks and bumps which they'll generally stand up to just fine.