I'm just finishing up A 4*60 right now, just ran out of one color so can't get it done yet, it is assembled though, just need to cover the front half of the top of the wing and do the trim. Great build by the way, very easy kit.
Wing sheeting is just for added strength for the crazy fools out there that like to pull high G's and odd stuff. I don't know anyone like that but I hear things.
Flaps?? No, just not needed, not even A little bit. However:::: if you have A nice big radio then by all means have fun with it. Flaperons and spoilerons are a kick to play with, you will have to add that second servo in the wings but it's well worth it. If you don't have A big radio then just leave it stock, it's really A fun plane no mater how you set it up.
The one mod I would make though is sheeting the top or turttle deck, either with 1/16 balsa or some 1/64 or 1/32 ply. The reason is you usually put the plane in A set up stand to assemble and you can break those nasty little deck stringers.
Don't ask how I know that!! Not to worry, A long CA tip and A stick to push it back into shape and it's good as new.
I have flown the AT-6 giant and there is nothing to even compare A 4* to the T-6. The T-6 will come in time but not any time soon. There are some nice looking 40 size T-6 ARFs but I haven't tried them. They would maybe be A good choice for your third plane if that's what you are shooting for.
Myself, it was the P-38 but over the years I have really backed off of big twins. Still haven't even flown A 38 yet but some of the electrics look pretty good.
For now enjoy your 4*, they are A great flying machine!!