I like all the 109's, including the E4. The K4, as others have said in the past, is among the best non-perked aircraft in the game. I actually think I like the G2 better than the F4. The G6 and G14 are fun but I tend to lean toward the G2 and K4 respectively instead.
I always take 100% internal fuel with no drop tank. It's more than enough.