And make the 4wt a 8' or 8'6. Fishing SNP is going to be nicer with a 7.5ft rod but that's too short for some of the bigger waters that you want to mend the line and high stick to keep the leader off the water.
With that said, the rods I fish most are 6wt's for small bass and bluegill. 8wt is really heavy, but it makes throwing large bass bugs, heavy sinking lines (shad fishing) and fishing for stripers or musky possible (but undergunned). I personally would start with whatever rod was going to get the most use and then add a different rod when needed. 9' six or seven weight for warmwater and then add a 4wt when needed. Fishing South Central PA's limestone creeks usually want a longer rod. SNP brookie fishing a shorter one. I know you want to plan out the decision now, but it's hard to know what you'll be fishing for until you start.