Hey Deus, hope these lights are working for you. according to the website below for optimal coverage based on 5spectrum LED in a 3'x3' space you need a 180w & 350w for 4'x4' (5spectrumLED). I also read that it might be better to have several lights rather than 1. For example 2 x 90w will improve the coverage of a single 180w...although the difference probably isn't worth the hassle. Im looking to purchase LED also & the cheapest guys I found with a good variety are a company based in Canada I think but not sure if they're legit to be honest as there isn't much info on them (link below). I could send you some more seemingly 'established' but more expensive LED suppliers if you like.
You want good and responsible price to cover 4x4. Either two area 51lights, 190s or 150s, or two onyx bloom. You won't find anything better at the moment for the price short of DIY.