[IAmBatman]

One thing about Marcus is that if they're using his soldier enhancement, they're not getting their all Valiant bonus, since Marcus is Disciplined. So basically they can choose to have one extra move or one extra defense die.
It seems like the AE (or anyone with 8 or higher range, really, so Syvarris, DED, and Q9 with his normal attack fit in here) has a good chance at taking out their points worth. The secret with the 4th Mass is to shoot them from out of range and make them move. That way they lose their attack bonus and possibly any height advantage bonuses. You have to make them play on your grounds, not vice versa.
Mimring is another one that might be worth taking a shot with, just don't get greedy. If they want to attack Mimring, make them move first, so keep him out of range and hit them one at a time. If they don't have the Valiant bonus and you hit them before they can get height advantage, they should go down fairly easily as individuals. If they have Raelin or someone else backing them, well that's just more points you can throw at them.
For a large group of 4th Mass, Drake makes a good clean up figure, because he can get there without taking shots, and he can kill them (though only one per turn) pretty easily with 6 attack die.
The real secret is just not to camp out within 6 spaces of these guys or let them get height advantage, though. If they're not getting bonuses, their base stats aren't that rough, they could even be considered mortal. So keep them moving, moving, moving!
Actually, since the 4th only have 6 range, the Krav should be a decent counter for a couple squads as well. 7 range and only needing 1 shield to block whenever one of the Mass rolls any skulls with their 2 attack dice should allow the Krav to stick around for long enough to get 100 points worth of kills, unless you're just having bad luck with rolls. Again, though (even if it matters less when you've got Stealth Dodge), don't let the 4th Mass attack with 3 die. It increases the chances they blank out, making defensive whiffs from the Krav less of an Achilles' Heel.