Four adult leos in a 15 gallon sounds unreasonably crowded, but its possible that they have more floor space than I think. I've kept a group of 4 in a 20Long.. ended up with dominance problems until it was reduced to a trio in there. Oddly enough, it was one of the females that was the main "bully".
I have used rubbermaid style boxes for laying, though with vermiculite rather than peat moss. I don't know of any reason not to use peat moss. I don't fully cycle my leos, but do have some off their under tank heaters for several weeks prior to adding a male. I'm in the San Francisco bay area (California) though, and the weather is now in the 70s at night. Maybe 60's but I doubt it.
Last year mine bred beginning in mid-march. This year they bred earlier by a month. I plan to try several in January .. would love to have some breeding now, but the area they are in gets a fair amount of natural light and I doubt flourescent lights will override it.
Hope this was of some help. Do consider a larger tanks if you plan to house 4 in them though, please.
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Andrea A. 