It really makes very little difference whether you learn to fly cast with a 4 wt rod or an 8 wt rod. Yes, the 8 wt is a lot more powerful than the 4 wt; but the casting mechanics are the same. It really isn't any different than someone learning to spin cast with an ultralight rod or a medium or medium-heavy spinning rod.
The bottom line is neither will be harder or easier to learn with. Get the one you will use the most first, learn to cast it, and then fish it as often as you can. Then a few months later (or a year later) get the other one. It sounds like you have decided to get two rods and will be using the lighter 4 or 5 wt most, so that is what I'd get first.
Just do yourself a favor and avoid the K-Mart (or Wally World) specials, which will hinder your learning how to do it and your fishing pleasure. Instead, get a good, decent casting set up from a fly shop or catalog house (Redington, ECHO, TFO, Lamiglas, and St. Croix, and even Cabella's have rods under $100.00 and complete outfits for $150 or thereabouts that will last you a lot of years and be good casting rods, unlike the very poor quality and poor casting K-Mart specials.