Season 4 had the best season finale I think I've ever seen on TV, with the start of the new agency and the final shot of all of the partners (and key supporting personnel) in a hotel room showing the guts to go it alone. It was awesome.
For season 5, I predict Don not getting married to his secretary and Joan's husband getting killed in the war, which makes room for her to get married to Roger Sterling (since it's his baby she's carrying).