Thanks for all the advice everyone. That's why I love this forum. I can get a range of different opinions on just about anything.

I think I'll follow Dennis' advice and stick with the .40 in the 4* for now. If I think I want more speed/ power, I may consider a .46 in the future. That way, I'll have an extra engine laying around that I "just have to get" a new kit for.

Again, thanks for your input everyone!