Alright!  I picked up the Argus Spartan about two weeks ago, Dare shortly after that, and most recently the Wetwork Spartan started rapidly flickering in & out of stock at Amazon and flipping between retail & scalper prices.  Fortunately I happened to browse through one of the times it was in stock at a reasonable price, letting me finally finish the collection!

Video: Mega Construx Halo Heroes Series 4 Dare, Wetwork, Arestor & summary