You can take on pretty much anything Act 1 has to offer at level 4. You should be level 5 before you leave Act 1.

There are some other things you can do in Act 1 as well to get XP. Spoilered out of an abundance of caution, I tried to give vague hints for each one.

Druid Grove:
Rifle through Kagha's personal property.
Check the passages behind the jail.
Go up the hill behind the Idol, towards the coast.

Blighted Village:
Talk to or fight the goblins here.
Find the apothecary basement.
Explore down the well.

North of Blighted Village:
Recruit Karlach and do her little sidequest.
Fight the gnolls.

Waukeen's Rest:
Rescue people from the fire.
Find the place where that caravan shipment is supposed to go.

You can get into the Goblin Camp without having to fight if you're clever, and then there are a lot of opportunities for XP there.