L.N.
if you don't teach the 4 yr old to sleep in her bed pretty soon you will have the 2 year old climbing up your bed.
i have never coslept with my kids in my bed. when they're sick i lie in their bedroom floor so i am right there, otherwise i don't bring them with me. mainly because my hubby snores to the point people in china are complaining :) so i know my kids wou;dn't have a good night's sleep. also i am afraid they'd fall off or what not.
what you do though at this point since she clearly thinks that's her sleeping place. well maybe one weekend, get up have breakfast with ehr and tell her this story that from today she will have to sleep in her princess room because she's a big girl now. tell her you need her help to decorate her princess room, maybe take her to some place for her to choose something, like framed art, or a nightlight she likes or what not. something small yet let her be in charge of choosing. for example tell her her princess room needs a nightlight so you need her to help you choose a nightlight. you get the point.
i did that when we first moved her and i was going to transition my girls from sleeping in separate bedrooms in cribs to sleeping in one bedroom in toddler beds. i let them choose the stuffed animals that from that day on will be sleeping with them. i also got a gate for outside their bedroom door, not because i was afraid they'd come to my bedroom but more because the stairs are right there. but anyway it worked for us. they have since forgotten the stuffed animals but the routine has been set.
so maybe give it a shot. it might work
good luck
vlora