I am no expert in 4 character domains but I think there isn't a market for them (unless it is something special that makes sense) so my valuation is $0-$15.
Thanks Kostas , I appreciate your answer . There is a french engineering company named a.o.2.a , but they have a crappy website name , I think ao2a.com is neat !
@Kostas is correct. There is no value to random 4 character dot coms without meaning behind the characters. If there is a company that uses those 4 characters as a brand you have a chance to sell it to them, but you need to make sure they don't have a TM first. Also, unless they REALLY want it and you play hardball, you wouldn't get much for it since there isn't any other demand for the domain. In general, it is $0 or regfee...
Thanks a lot @svede , I really appreciate your help . You guys helped me change my opinion about my domain name which first appeared to me as valuable since it's 4char and old enough (since 2001) , but now I don't see it as I did before !