Unless you want 802.11ac, the 4th gen AEBS should continue to work fine for you. I'm using the same one myself, having upgraded to it a few years ago from a second gen. I don't think Apple is putting much effort into these anymore, however, so unless you have some reason to prefer an Apple router (e.g., AirDisk, Time Machine via the USB port--only officially supported on the 6th gen, BTW--or the desire for Server.app to be able to manage some settings on your router automatically), I'm not even sure I'd consider it anymore. Additionally, with the latest generations, you'd actually lose some functionality in that AirPort Utility 5 doesn't support them, only AirPort Utility 6. If you don't plan on changing many settings it won't matter much to you, but many things were easier in v5, and v6 downright eliminated some (albeit probably less used) functionality that I find useful (e.g., transmit power and the ability to see more than two DHCP reservations at the same time).
If I ever replace my AEBS 4, it's probably going to be with something non-Apple, probably something I can load an open firmware onto (though 802.11n routers that support that seem to be rare, to say nothing of 802.11ac). EDIT: Actually, DD-WRT-compatible routers are easier to find than I thought, including the Linksys WRT AC1200, which is probably what I'd buy if I had to get a new one right now. A bit cheaper than Apple's options to boot.