Dan has it right....BUT....You said you didn't want the AC2 because of a 3wt rod. I had two Abel TR3 reels: 3.25" diameter, standard arbor, and .75" spool width. They held a WF4 line nicely, but I wouldn't go any smaller. Abel's site claims that reel could hold a 7/8wt line. I'd like to see that! Abel is also saying that the AC1 will hold a WF3 with 55yds of backing. You might be able to get a WF4 on there with a few feet of backing just to attach the line to the spool.
You could get the AC1 with a standard arbor for your 4wt, and large arbor for your 3wt. You could also buy the large arbor Abel for the rod you'll use most often (AC2 if it's the 4wt), and buy a Cortland Retro for less than half the price of an Abel spool for the other rod. That gives you two reels in the proper size, and without compromise. You also save enough to buy premium lines in both sizes...or put in the bank, etc.