Your 4set would be better as the helm alone wouldn't equal to a 4% gain in your obliterate damage.
Generally; regarding rather or not you should use a tier bonus or not; if the tier piece is only a patch old it will be better using it (unless it's something worthless like t11 4pc.) rather then an off-piece.
Well, the helmet still wouldn't be an upgrade over a 25% str 1 min cd over a 20% str 1 min cd.
Helmet isn't an upgrade in either situation.
it's actually pretty similar. also depends on how well his pillar of frost usage is.
just off his battle.net page: str gained from the extra 5% of pillar: 802. divide 802 by 3 to get average str per minute: 267.3
what he gets from the helm: 210 str + 80 extra str from better gem socket = 290 more str + more secondary stats.
also gives 58 more str during a pillar of frost over his old gear so 19.3 additional str.
i'm sure if you sim them they are almost identical. This doesn't include any damage boosts or stacking cds or anything else though.
and since I was bored: 502 helm: 97932.5
t14 lfr 4piece: 98221.6
used: 50000 iterations, low latency, length 450s, vary length 20%, fight style: light movement, num enemies: 1, player skill elite
so yeah, if his pillar of frost usage isn't great then the 502 helm will be better. if it's good then the 4piece is better, but not by much.
and when I switched the sims from player skill: elite to player skill: good the 502 helm > t14 4 piece.