Well I think Peter and MJ reached adulthood in the last scene of SM3. So to see them cope with an engagement and the approach of marriage but see how marriage has torn the Connors family apart and Peter's driving need to save Curt Connors to know that a superpowered being does not always have to destroy his family....could work REALLY WELL.
I do agree that after that there is not much more room to take the Peter Parker story movie-wise but two movies of filler with more love triangles (be it more Gwen or Black Cat) would just seem that...filler until they get hitched in SM6. I think pay off a great Lizard storyline in SM4.
Besides, I personally doubt the franchise will go all 6 planned films. I predict around 5, whether Tobey, Kirsten and Sam come back or not. They can do an emotional and SM2-quality-sized fourth film and then use Carnage as a last ditch effort for public interest in SM5. After that, what is left that will keep audiences coming back, though?