I have two children a girl that is four and a boy that is five. we have very limited space and I am wondering if it is still approiate for them to share a room

My children shared rooms at this age (opposite sex). In fact they were sad when we separated them.... In the UK there is age for separating opposite sex children if you are in council house accomodation/ receiving housing benefit... so eg.... if you have 2 children of opposite sex... I think it is until the girl is 8 (I may be wrong about the age), you are only entitled to a 2 bedroomed house, as the expectation is that they would share... when the girl gets to 8 or whatever it is... you are entitled to 3 bedrooms....
So at 4 and 5... it's absolutely fine.

I grew up in one room with my brother all the way till leaving home. I think this is how it is in most countries. Actually the whole family is often in one room. Most countries don't have more than one or two bedrooms. I think we are becoming gender crazed. Children are fine and will tell you when they need their own room... like when they start having sexual feelings in puberty. Nothing before that.