Originally Posted by Moogul

Yes, 4T6 means 4-pieces of Tier 6, and in particular the set bonus of that. You'll find that notation used a lot on these boards, such as 2t4 (two piece tier four), 4t5 (four piece tier five) etc.

I'm not sure particularly what you mean by 'what's the logic behind that?', but we were discussing whether wearing 1 'subpar' piece of Blacktemple T6 instead of a 'better' Sunwell item would give higher total dps, due to the 4t6 set bonus.