Yeah I'll have to consider the 4 weight. Although it is tempting to think about getting a #5 and #3 like you say haha.a 9' #5 is a heavy rod for small rivers. OK for small still waters, but so is the 8'6" #4.
I just think you have more bases covered with a bigger gap between sizes than just 2 line sizes. For small fly imitative fishing the lighter rod will be perfect, river or lake, even BIG lakes and you have the #7 for anything with bigger flies or longer range. Way more versatile methinks. Or just get a 9' #5 AND a 7'6"#3