Yup one warlock will be swapped out as needed.

I was thinking of the 1 pally, 2 shadow priest, 2 locks approach but for some reason shadow priests just don't appeal to me. I played a while ago and don't feel like playing a 2nd one. Though a shadow priest's shadow word death does the dot equivalent of corruption and curse of agony in less time when the lock / priest have equal spell damage.

Plus with 4 warlocks, each affliction on the target increases drain life by 5% (4% in 2.3) ... 4 locks x 4 dots each (corruption, siphon life, shadow embrace if it stacks, coa) 16 affliction dots on target ... 16 x 4% = 64% ... effective cap 60% healing and damage bonus to drain life. With enough spell damage and improved fel armor that would heal for nearly 500 per second. IMBA!