Talking out loud!

So me and my boyfriend have been together for 4 years and we have lived together almost a year.. And I desperately want him to ask me to marry him.. I've dropped hints but he says he's just not ready for that. We are realativly young (20) so I understand.. But what is the harm in asking.? I found out recently that he was gonna ask me for our anniversary asked my parents and everything but when it came to it he got cold feet... He really is a good guy and we have a pretty good relationship but I'm scared he may feel differently.. He talks about our future together and plans like we will be together forever but still...