Now it is time for you to fight and possess the Sorcerer. Fortunately, it is right nearby. After you return from the third dungeon and deal with the Bouncer, crouch down and shoot out the lower portion of the left wall. Crawl through and you will find an Amulet, and the Sorcerer waiting on the other side. As a Gunner, it will take relatively little effort to fight and defeat the Sorcerer.
Note that if you do not have an abundance of lamps (at least 5), you may wish to reenter dungeon 3 and complete the first floor of it over and over until you have a sufficient amount of lamps.
After climbing all the way back up the ladder along the right side of the fortress, allow yourself to fall through the pit that you will arrive at. Head left when you land. There are two statues nearby, both of which contain magic books. Collect them as you need at least three before you can enter the fourth dungeon.
After you collect the books, continue left over the small pyramid of blocks, but watch out for the two fires on the other side. Remember that as a Sorcerer, you do not need to crouch down to hit low enemies. Collect the energy item, then return to the top of the pyramid to climb up the ladder above.
When you fall down to reach the bottom of the next ladder, you should fall to the right, in order to avoid the caterpillar below. Climb up the ladder, and head to the right to climb up the next ladder. In the region above, you can collect two more magic books, which will get you to the requisite three books that you need to break blue blocks. Return down the ladder, and head to the left. You will now be in familiar territory, facing off the the Blaster you fought earlier when you had the Blade Army body.
The entrance to the fourth dungeon is one floor above this Blaster, to the right. Avoid the Bouncer and use your magic to remove the blocks surrounding the entrance.
Aside from the darkness that occupies the two center floors, you will probably find that the offensive powers of the Sorcerer make this dungeon fairly easy to complete. Just be sure to fire enough in advance to destroy any enemies, and shoot through the walls if necessary.
You should make it to the guardian of the idol with relative ease. When he appears, your best bet is to jump back down the ladder to the right that you climbed up to reach him. The guardian will not follow you down, and loop in a pattern when it dives down in front of the ladder before retreating. As it dives down, jump up the ladder shaft and fire at it. It will be defeated with only two hits. Then you can proceed to the back and collect the LAHRO idol. (Don't be confused by the LAHRO's appearance as it blends into the wall quite well.)
There is only one difficult point on your return journey to the fortress. A flame on the floor below the boss will make it tricky to drop down to the ground from where you can attack it without getting damaged. Try to drop down and hold right to guard against it, and fire as soon as you can.