how do you raise Rosie's level 4?
Did you do?
Come in her room during the first part of daytime and talk to her.
Ask her about the swimsuit.
Wait until the next day's morning event.
Talk with Rosie about going to the beach and complete the event.
During the second part of the evening you will get the next event (After this event next events bellow at lvl 3 are changed as well).
Talk with Rosie when she in the shower.
Come in the toilet when she in it.
Go with her in to the gym.
Now you need to increase the relationship to 100%.
When you complete all of the above you get access to the level up event. It starts automaticaly at the daytime.