I want SM4 to end with the inevitable proposal.
Then SM5 begins with the wedding. Not long after, Aunt May dies, telling Peter on her deathbed that she can finally rest in peace now that Peter and MJ got married. Sometime later in the movie, MJ tells Peter that she's pregnant.
MJ goes into labor at the same time that -insert SM5 villain- has prepared for his last stand. After a triumphant victory over -insert SM5 villain-, Peter feels like a million bucks. The city cheers him on as he swings to the hospital in his tattered costume. There, he makes a quick change and waits in the waiting room. After a short montage of doctors huddled around MJ and Peter pacing anxiously, the screen goes black and we hear MJ ask, "How's the baby?"
The screen stays black for the next couple moments until we see Peter in the waiting room. A doctor comes over to him and whispers into his ear. The audience cannot hear what is said, but Peter's nervous grin turns into shock and horror. As the doctor walks away, Peter breaks down.
Black screen once again..giving the audience time to piece it together (if necessary). When the screen comes up again, Peter is at MJ's side. They both have swollen, red eyes...and MJ isn't fully "together" just yet.
Peter asks her where they go from here.
Cue inspirational speech from a shaken MJ. She tells him that they will do what they've always done and they will get through this. If not, "everything we ever were will mean nothing." (Come on, the line HAS to get used!)
Throw in a montage of clips of Peter and MJ in the hospital. Show them crying, show them consoling each other, show Peter catering to MJ. Finally, we stop as the couple is asleep...Peter is asleep in MJ's lap. Sirens can be heard in the distance. They both wake up...MJ says in a feeble voice (the kind you get after hours of crying) "Go get 'em, tiger."
Peter performs a quick change and leaps out the window, heading to wherever. Cue final swing sequence.
It may not be the happiest ending, but it's Peter's life. Plus it shows how strong the relationship with MJ really is.