The math, in case anyone's interested:
4 Ultra/5s at 8 tons each, 32 tons total
2 ER PPCs at 6 tons each, 12 tons total
44 tons of TC'd weapons
Therefore you need an 9 ton/crit Clan targeting computer (or 11 ton/crit IS targeting computer, which is even worse). LRM 15s give the tonnage for a Clan computer but not the crits - only 6 crits between the launchers and ammo.
EDIT: Fixed the tonnage, making the whole mess worse.