NL25: What has this guy?

[neaera]

I'am feeling relatively comfortable with most situations at NL25, but there are still weird things happening.

Villain is a multitabling TAG (22/17/4), who seems to have a good reading ablity. on the other hand: he calls a MP raise in a 4way pot with ATo in the BB. don't really know how to interpret that.
My image is 18/13/2.6
Now this hand came up, where I am like 99.9% beaten, but have no clue what he has exactly. Could this be a bluff? Any ideas?

The cbet is so small because the board is the driest out there.

Poker Stars - No Limit Hold'em Cash Game - $0.10/$0.25 Blinds - 6 Players - (LegoPoker HH Converter)

SB: $29.50
BB: $9.55
UTG: $24.85
MP: $25.25
<font color="black">Hero (CO): $24.15</font>
BTN: $61.95

<font color="black">Preflop:</font> Hero is dealt A[img]/images/graemlins/spade.gif[/img] K[img]/images/graemlins/diamond.gif[/img] (6 Players)
UTG folds, <font color="red">MP raises to $1.00</font>, <font color="red">Hero raises to $3.50</font>, 3 folds, MP calls $2.50

<font color="black">Flop:</font> ($7.35) 3[img]/images/graemlins/club.gif[/img] 4[img]/images/graemlins/diamond.gif[/img] 9[img]/images/graemlins/heart.gif[/img] (2 Players)
MP checks, <font color="red">Hero bets $4.00</font>, MP calls $4.00

<font color="black">Turn:</font> ($15.35) 6[img]/images/graemlins/heart.gif[/img] (2 Players)
<font color="red">MP bets all-in for $17.75</font>, Hero folds

Pot Size: $33.10 ($0.75 Rake)

I can't call here, unless I have the monster read on him, thats understood. but what hands does he do this with? A set hoping, that I'll call with an overpair? SCs? smallPPs? Air?
What hands to you call this with? I can't imagine that any onew pair hand is good here.