I think a true 500w panel from a good company could handle a 4x4 just great. With the best LED panels I think you only need 30-40w per square foot of floor, and a 500w on 16sq feet is like 31 so it would be very appropriate in my opinion. Even a single 500w panel from a good company would wipe the floor with any 600w HPS, but you could do even better in that 4x4 space if say you bought two 250w or 3-4 150w or something (the more the light is distributed the better). To cover a 4x4 tent will cost you up to $3 per w, so expect to shell out $1,000 - $1,500 to get about 500w of good LED panel/s in a 4x4. I wouldn't ever go more than 65w per square foot of floor, I have a true 395w in my 2.5 x 2.5 x 6ft so that is about 62w per sq ft and its definitely excessive