Only Raid L4... What Pokemon needed for offense?

For Snorlax, T-Rex, I would definitely use Dragonite DB, DT with H, O

For Lapras, 6 Flareons with FS/O

Basically, any heavy hitters DPS-wise that have moves that either hit neutral or SE and don't have weakness to those 3 mons' moves will suffice. High DPS is key especially soloing it

if you are looking for offensive? against the tier 4 raids?

Dragonites and Tyranitars. Those are the only maxed out attackers I need for level 4 raids unless it's against a Lapras boss.

Eggs will get crushed by Lick and HS Snorlax bosses.
Machamp will get eaten up by Zen Headbut lax bosses
Those two will only do well against Lapras.

You need high resistance types and high DPS SE moves into bosses because you're not guaranteed 20 trainers will show up. You may end up with 3 or 4 trainers only and y'all better have some heavy hitters into these bosses.

I soloed a 18k-20k Raid boss yesterday with 6 Dragonites and there was plenty of time left. Most of the raids in city have plenty of players joining in.

Machamp for sure. Dragonite, Tyranitar, Vaporeon works fine. Even a Pound/Hyper beam Blissey will perform well in groups.
Pretty much anything you invested in that has a type benefit.