My composition includes an elemental wizard and a marksman + the other two.
My characters are all lvl 4 as well. What am I not considering?
I did this at level 4 with the same party composition. Initially it felt like a real road block but then I thought of a cheesy but fun strategy:
1) First I picked some ooze and oil barrels elsewhere. (Did you know you could put them in your inventory? :))
2) I unchained my marksman from the rest of the party, sneaked as close as I could and jumped (Tactical Retreat) to the ghoul carrying a couple of barrels with me. After dropping the barrels on the ground I used remaining APs to get some distance and flee from combat.
3) Used one pyramid to rejoin my party and again sneaked in with the marksman. When he was noticed I used another pyramid (on my mage). This put my party was a bit closer to the enemies than just running towards them so I didn't have to waste APs on movement.
The rest was easy or at least nothing special. Detonating barrels with Fireball takes a good chunk of the boss HP and kills two hounds instantly. I had Teleport on both mages which also helped immensely (it's really a must have spell on Hard).
So it's really doable at L4 without any OP talents like Leech but you may have to bend the rules a little.