PS4 - 19M (As much I would like it to, I don't think it'll top this year, although may get closer to 20M. If it betters 2017 and gets what will have to be 21-22M to top it, I'll be very surprised and impressed in equal measure).
NSW - 18M (Should see a significant increase on 2017, again, but not yet hit the heights of 20+M just yet. That should come probably in multiple years down the line. If it does reach more towards 20M, I will also be surprised and impressed, but should be down to one or two big hitters being released).
XBO - 8M (This is my most ambitious prediction of the lot, as I think the Xbox has the greatest chance of falling flat and only achieving 6-7M. Should depend on discounts and if the One X can help boost total sales).
3DS - 4M (I generally have less to go on and haven't kept up with its general performance vs the other 3, so this is more a general prediction based on post before mine).