Well, the easiest option is to just add another two t5's in there on another ballast.

Something I think you should first do is forget about the whole watts per gallon "rule" it's very general and typically wrong, especially as you get into more efficient lights. For example, my LED setup only runs 160 watts, I have more light output than my 600 watt previous setup.

There are a few off-the-shelf LED type items that might be useful, but if you are going to go partial LED you might as well go full LED. If you're going to go full LED, the only affordable (and by affordable I mean less than $300) option is DIY, but it's pretty labor intensive.

If I were in your situation I would probably go with another 2 bulb t5 retro kit.