97 Expedition Fouling out Plugs

I had the tuneup done at 80k because that's what my mechanic said I need. Ok no problem till my check engine light came on again took it back to the shop he said that the no. 4 plug had fouled out. He replaced it and about a week later it rained, my check engine light came on again. Took it to the shop no. 4 plug fouled out again. It took me about three times of it raining and my plug fouling out that I noticed a patteren. I asked the mechanic if there was somthing that they had done, to cause this and he said no. To cut it short, has anybody had the same problem with no. 4 plug fouling out when it rains? I did not have this problem before I took it in for a tuneup. Help