Congratulations!
I actually like your idea of finding the shirt in the laundry while he folds.
Question is: is he going to notice the "new" shirt when he folds it.
My husband is pretty perceptive, but when folding a load I doubt that he would notice it, thinking it was just another shirt to fold.
Maybe figure out a way to get the other 3 kids involved, if their old enough, to share the surprise together. Have them each were shirts with one word on it, including you: MOM IS PREGNANT AGAIN! You can have the word pregnant on your shirt and come out last. Then present him with his shirt!
I know that's kinda silly.... But this way everyone can have fun and be involved in sharing the good news!
Good luck with your pregnancy and have fun with it! :-))