Your wife has excellent taste, cichlids are amazing fish - not only beautiful, but with bags of personality. Your water being soft is perfect too, as South American cichlids thrive in this type of water. I think your best option would be dwarf cichlids, which don't get too large, and also don't eat plants. The options are these, all of them are great options, so if I were you I'd go to your LFS and see what they have (all except the last option are commonly available) and see what really takes your fancy. Ideally you would keep these fish in a pair for the least agravation, or maybe a harem. In a 4-foot tank you could keep some more, as long as it's nicely planted so they can have defined territories.
German Rams - these come in 3 colours - regular (which is multicoloured and very pretty), electric blue and gold. The only annoying thing about them is that they really prefer a higher tempurature of 28, which other fish will be uncomfortable with, so you have to stock the tank around their tempurature.
Bolivian Rams - these only come in one "colour", which is less colourful than german rams, but they are happy at more "normal" aquarium tempuratures, and you can keep them in a group. I used to want german rams due to the colour, but there is something in the eyes of the bolivians that makes them so beautiful and intelligent.
Apistogramma - these come in all sorts of colours, they're my favourite fish and are very funny as well as beautiful. The most colourful + not hard to find are: A. Macmasteri, A. Borellii, A. Cacatoides, A. Agassizii. Most of these types have amazing variants which are extremely colourful (check out agassizii tefe or double red). They will definitely breed in your water, which makes them more aggressive, but if you read up about it and don't get too many (a pair or a harem of 1 male 2 females works great), and research good tank mates for them, you will be fine. The females are less colourful, except when spawning where they go bright yellow (and in your water they'll be constantly spawning and constantly yellow I think).
Be aware that all of the above require an area of fine sand to clean their gills/eat from (for best health), and ideally build some caves into the hardscape, or get some seedpods/coconuts or piles of dried leaves to spawn in & create a territory around. They're all territorial, but with a nicely planted tank that's fine because what we think is beautiful is usually what the cichlids like too.
Lastly, Dicrossus Maculatus or D. Filamentosus - these are rarer small cichlids which require really soft water. They are smaller than all of the above and a few can (probably, if you get a good-sized tank) live with a pair of the above cichlids (they live in the wild with some apistogrammas). They've got a lovely pattern, and the males have extraudinary tails and a beautiful irridescence. The females are more black and white checkerboard-y, which is lovely too. If you keep them alone, you could keep them in a group of 6 with 2 males in a 3/4 foot tank.
