Last name didn't change after getting married.
I moved two sims in together and when they got married, the female still has the same last name she did before they got married, and their child has her last name.
Forum Discussion
In the Sims 3 there were some issues with spouses' names not changing sometimes too, but at least you could go to city hall and change it. I think a pop-up would be nice with just their last names being able to be changed (after all, getting married doesn't mean changing your entire identity), or maybe an interaction on the computer/phone to "file for name change" or something. I don't usually have a problem using cheats to change little things but sometimes full edit CAS glitches and sets everyone back to the beginning of their age so I'm always a bit wary to use full edit CAS.
