I agree about the summer birthday thing.
Personally I've managed to avoid having parties for my kids, but that's personal preference. I just think they get too crazy. I did have one for my middle child's 8th birthday- out of the 12 boys invited (all came!) he really only played with 3 or 4 of them, it was silly. Instead we've had sleepovers with 2 or 3 friends, or even just one, maybe take one friend to a hockey game or amusement park- it's still a special treat for my sons and there's none of the worries about reciprocating, mom cliques, etc.
Of course, my youngest is already planning his next birthday party (not till Feb. thank goodness!) and if he really has his heart set on a big party it may be hard to say no, but I'm hoping we can come up with a fun option that makes him just as happy- we've always managed that with his brothers!
But, to get back on topic- 4 or 5 kids is perfect for a bowling party at that age- they'll have a blast!