There's a lot I want to say when talking about SMT IV:A. I think it improves on SMTIV in almost every category. I think the story is better. The combat system has been refined and improved. The world map navigation isn't as cryptic and difficult to manage. You don't have to sit through the same terminal screen you just saw when you reach your destination after using the Terminal to fast travel. It goes without saying if you enjoyed SMTIV then you should definitely pick up Apocalypse. If you started SMTIV, but couldn't finish it because of the difficulty, the world map, the sometimes irritating design decisions then you should also pick up SMT IV: A. Do you have a 3DS and enjoy turn based combat? THEN YOU SHOULD PICK IT UP. The only people I wouldn't recommend Apocalypse to are newcomers to ATLUS games, and that's a shame because they've really nailed it with this iteration of the series.
The only reason why I say that is because I think your enjoyment of Apocalypse's story will largely depend on the investment you had on SMTIV. I feel if you really want to get the whole experience you gotta start with SMTIV first, and to go further down that line of thinking you should pursue the Neutral route in that game, if you can, on your first playthrough. SMTIV was the first game I picked up on 3DS. I was late to the 3DS party, but I can still say that game in particular is the one I think of when I justify my 3DS purchase. I was lucky enough to get the Neutral ending, by some insane miracle, on my first playthrough.
SMTIV's perceived faults, after spending many many hours with the game, are also oddly its strengths. For example, the extremely confusing world map is only really confusing at first. In hindsight, I don't feel like the map was really the main issue at all.. The complicated map is what lead to the depth and later the real enjoyment of exploring the world, or at least that's the way it worked out for me. The issue with the over world travel had more to do with the incessant interruptions fighting what usually were lower level demons. The difference between these encounters in SMT IV and Apocalypse is immediately noticeable. The glowing orbs representing demons in SMTIV would interrupt your travels every 3/5 seconds and would even literally chase you down. The irritation of this was usually the fact that they would end up being lower level demons that you already recruited/fused, didn't get much experience from, and overall felt like irritating busy work. It made trying to find your next location and deciphering the cryptic hints you were following incredibly irritating when they should have been intriguing. This issue is almost completely eradicated in Apocalypse and fixes most people's main complaint about SMTIV. Now that I've finished Apocalypse I feel like I could count the amount of times I was interrupted by lower level demon fights on one hand. If I wanted to avoid fights on the world map, and in general, I largely could. I wasn't always forced into fighting scrubby demons while I was really trying to track down a side quest goal. Furthermore, there's even an 'app upgrade,' a level bonus for those that don't know, that actually will make lower level demons completely avoid you altogether. I didn't purchase this app on my SMT IV:A playthrough and it still wasn't a problem for me. This adjustment alone really opens up the game to be enjoyed by most people more than its predecessor.
ATLUS also paid a lot of attention to the minor detalis that made SMTIV frustrating at times as well. One example, in SMTIV when using the 'terminal' to fast travel from one location to another you had to flip through the same screens when you initiated a travel AND AGAIN once you reached your destination. In other words, you'd be shown the same list of destinations once you arrived at the destination you just picked only seconds ago. This got incredibly old after playing for numerous hours and it made absolutely no sense to do it that way. It was a redundant/bad design decision. In apocalypse you pick a location and it immediately loads the destination so you can get on with it. I marvelled at this small change every single time I used the terminal in Apocalypse.