First, the term "feral" does not mean "outdoorsy." It means not socialized to be accustomed to humans. So taking kitty to the forest now and then won't make him feral. It's kind of risky though, since he might decide to wander out of your sight and get lost.
A 4 month old kitten is still pretty much a toddler. He does not get it. If he's playing in the dirt, you need to distract him and give him something else to do. Make sure he has proper litter tray and knows to use it. Make sure he has interesting things to play with, toys and such. You need to interact with him for at least an hour per day. It would be better for him if he had a buddy cat of the same age, since the two cats will want to play roughly the same amount, and keep each other company.
If he persists in playing in your house plants you should do the following. First, reduce the temptation. As much as possible, put the plants in places where he can't get to, or cover the most attractive ones with screen. When you see him in the dirt, pick him up and put him on his own toy. Start with speaking strongly to him and say "No!" If he persists, gradually increase the stern. First a shaken finger, and eventually give him a very gentle smack. Extremely gentle, just enough so he is aware you are angry.
You can also try a squirt gun, give him a little squirt when he gets into the plants. That's a bit risky though, since some cats will treat that as a game.
Keep in mind he is still a baby. You must be consistent. You must be loving. You must be firm.