Probably should have given this a bit more thought before posting, as on reflection it seems fairly obvious. Assuming negative values return nothing, (-n+4) yields [3, 2, 1, 0, -1, ...] for n = 1, 2, 3..., and so this should give the first three elements of a set (given 1 based css indicies). Hmm, yes, refelection; always a good idea.
Yes, that is probably true - certainly would make things more readily readable. I was just wondering about the use of negatives in expressions such as this, since I'd been reading about :nth-child and it wasn't mentioned. Although assuming it does work as I'm guessing above (haven't actually tested it yet), it does seem quite a nice solution to selecting the first x rows.