What was the name you used when you tried to share your livery? The term GT4 ik right off the bat is a blocked word. So you are going to have to troubleshoot and find different names.
A list of some of the found Blocked words
Sometimes a word can feel like it doesnt make sense to be blocked but it could have a different meaning from another game… or mean something darker. Since the filter itself is created by Xbox its kind of out of Forzas Control