remove characters that are different from ABCD in a file

[papori]

Hi guys!
I have a text file, and I want to delete all the characters that are not A,B,C and D.
For example if my file contain this 2 lines:
ABCDEFGHIJ
GHKLABC

the new file will look like:
ABCD
ABC

I am looking for an easy way.. ideas?

Thanks,
Pap

[cfajohnson]

If you remove all characters other than A, B, C, and D, that will not
be the result; it will also remove the newline:

tr -dc 'ABCD' < "$file"

The result will be:
ABCDABC

What you want is:

tr -dc 'ABCD\n' < "$file"