SQL & PL/SQL :: Show Last 4 Digits

I have a requirement where I have a value stored in varchar2(12), and no matter what the size of data, I should mask all the characters except the last 4

eg: - 23115645

after masking should be - xxxx5645,

create table test1(x varchar2(12))
insert into test1 values('34567745')
/
insert into test1 values('438765311')
/
insert into test1 values('1112345')
/
commit

[Code].....

y should store value of x in such a way that no matter what the size of x is (7 characters or 9), we should see only the last 4, but the remaining should be masked with xxxx

I tried the above update, but using substr, i am able to get this if x has 9 characters

not sure how to achieve this if x has 7, or 4 characters