[SPOILERS]I see people struggling to find xp early in the game. After about 8 playthroughs, I have determined the following steps to reach level 4 BEFORE leaving Fort Joy. I won't go into party building or earning gold, as those are covered elsewhere. For this you need at least 1 point in thievery, and look to buy any items that increase it. You don't absolutely need to do them in this order, but you do need to be a ruthless bastard. Here it is:
1. Kill the 2 Viscous Voidings near the start.
2. Dig up and kill the fresh buried corpse at the north part of the map (need a shovel or lizard in party). Pick up the tong on the south part of the beach (for equipment repair).
3. Dig up the pouch near the entrance to Hidden Alcove.
4. Take the vine to the upper level of the Hidden Alcove. Talk to the girl. Kill Unnis.
5. Just inside the main gate, talk to Fara (need a human or dwarf in party, if you don't want one, invite Lohse nearby then dismiss her after this). Offer to help Fara, then talk to Jeth, then to Fara again.
6. Confront Burro and Kana and kill them without aggroing guards or nearby NPC's. Follow the elf you saved to the Cavern.
7. Just before the Cavern, cast heal on Dain.
8. Enter Cavern and play hide-and-seek with Mody to reveal a mound to dig up a hatch.
9. Down below talk to Lord Withermore. You can remove his spear with sufficient strength (Kalias above sells a strength potion). Or you can attack the spear and destroy it.
10. Go to the Shine of 7 and select the option to reveal the hatch.
11. Go down and pick lock or break door to Vasyna. Go past her to just enter the Underground Tunnel, then turn around and come back.
12. If you have level 2 thievery, pick the lock to the door to the Main Floor.
13. Go through the door and go straight (do not go the other way at this point). Pull the switch on the left and take the hatch down.
14. Destroy the jar on the back left (only!).
15. Return to Lord Withermore to get his belt.
16. Kill Saam and Baladir near the Fort Joy kitchen area.
17. Kill the 3 card players on the upper level.
18. Talk to Magister Yarrow.
19. Go to the beach past the Cavern on the left.
20. Kill Migo and take his ring and armor.
21. Go back to Magister Yarrow and give her Migo's ring as proof.
22. Follow Magister Yarrow back to Migo's beach. Kill her and take back Migo's ring.
23. Dig up the nearby hatch to the Underground Tunnel.
24. Go down and kill the lone fire slug.
25. Talk to the elf in the cage near Boss Griff. Offer deal to Griff to find his stolen goods.
26. Go to the beach to the right of the cavern and convince Stingtail to give you Griff's orange.
27. Return the orange to Griff and narc on Stingtail.
28. Return to Stingtail and wait. Silence will fight with Stingtail. Kill them both when they are weak.
29. Kill the 2 magisters near the Beach Shrine.
30. Kill the 2 magisters by the main gate entrance.
31. Kill all NPC's in the Cavern.
32. Hopefully you are level 3 and ready to kill the crocodiles on the beach and get the teleport ring.
33. Go to Gawin. Finish his quest or kill him and take his robe.
34. Go further into the Hidden Alcove and kill the void touched turtles.
35. Go to the balcony behind the tent to the right of the gate heavily guarded by magisters. Teleport your lock picker up to the balcony to lower the ladder.
36. Pick the lock to Orivand's Chambers. Pick up the teleport book and loot. Note you can TP outside his window for another chest.
37. Back to the Underground Shrine, go slowly towards the Magister Houndmeister (but do not engage). TP into the room blocked by rubble to the left.
38. Return to the Beach Shrine. TP up to the broken bridge and go up the hill for xp and loot.
39. Kill the magister and his dog patrolling the beach inside Fort Joy.
40. Kill the frogs past the barrels inside the cavern. Go to the dock for xp, and there is a chest you can TP to.
This should get you at or very near level 4. If you come up short you can pick off some silent monks on the Main Floor. Also you can try your luck in the arena below the hatch near Boss Griff. After the arena, at some point you can also take on the group where you got the arena quest.
Did I miss anything?