Sadly, it seems that MekTek have stopped offering a free download of MW4: Mercs. The cause of this appears to be the usual legal clusters that plague Battletech. The new MekTek forums have the details. So to all of you that still have the MW4 files but no disc, backup the hell out them. There's no indication that MW4 will be put up for download again like it was at MekTek and it would be a terrible shame if this great game was to just fade away.
