I can't see full support class on 4-man content, his buffs would have to make up for the lack of one DD....if they do that (huge buffs to make up for the lack of one DD) then in 8-man content said buffs would be OP.
Full support class or 75% support/25% dps classes usually work better in games with 5 or 6 men groups (1 tank - 2/3 dd - 1 support - 1 healer)