I preferred 4 over 5 by a fairly long margin, although I played 5 before any of the DLCs (which would both cost $50 if they weren't on sale (and the most recent one is still $45 (in Australia))) were released so it might have become better. My main complaints (note that I haven't played 5 in a very long time, so my memory might be a bit wrong) were how difficult it was to take down cities (IMO a city's defense should be from what you've garrisoned inside it, it shouldn't be as strong as 3-5 units by itself), diplomacy being made quite simplistic compared to 4 (IIRC you aren't even shown what's effecting your relationships), and lack of mods (which is only partially the fault of the game itself, although I hear they didn't exactly make it easy for modders to do anything they wanted). I did like that it tried out hexes (IMO better than square tiles) and only having one unit per tile (probably a change I don't want to see in further Civs, since it means you can't have armies as large as in the previous games just from lack of space, although either making it a higher limit (5-10/tile?) or making each tile represent a much smaller amount of land, splitting each hex into 6 (for example, and this is probably the only number to work well without being needlessly complicated) for movement while keeping cities the same (so they would operate on the groups of 6 hexes). Maybe bring back something like the zone of control system in the older games too, so if you have a long front with units on each tile the enemy needs to break at least 3 of them to advance into your territory. Enough rambling, good thing it has a demo, if only 4 had one too so people who haven't played Civ before at all could examine each one to see which they preferred.
Of note is that the rest of the Civilization series is also on sale. If someone ends up buying 4 as part of not the complete edition (I have no idea why anyone would do that, since it's the cost of 4 and the important expansion, but not only gives you those but also the spinoff game and the other expansion) then don't get Warlords, since everything that's in that is included in Beyond the Sword, except for 6ish mediocre scenarios (and support for mods built for Warlords, but most of them a for BtS anyway). But buy the complete edition while it's on sale if you want anything to do with Civ 4.