Replace all characters except the first four characters

With GNU Sed,

md5sum input.txt | sed 's/./x/5g'

This simply skips substituting the 4 first characters of the string and performs the substitution for all other characters.

A POSIX alternative with Awk (although there is probably something simpler),

md5sum xad | awk '{
  four=substr($0, 1, 4)
  rest=substr($0, 5)
  gsub(/./, "x", rest)
  print four, rest
}' OFS=""

POSIXly (I think), you could use a sed loop to repeatedly replace the first non-x character following the 4-character prefix:

$ md5sum input.txt | sed '
:a
s/^\(....x*\)[^x]/\1x/
ta
'

Replace [^x] with [^x ] if you only want to do the substitution in the first field (the checksum).

With perl if GNU sed isn't available:

md5sum input.txt | perl -pe 's/^.{4}(*SKIP)(*F)|./x/g'

^.{4}(*SKIP)(*F) will prevent replacement of first four characters

|. specifies the alternate pattern that has to be replaced

To change only the checksum:

md5sum ip.txt | perl -pe 's/(^.{4}|\h.*$)(*SKIP)(*F)|./x/g'

If the md5sum output starts with a \ (for ex: if filename has a newline character), then you can use ^\\?.{4} instead of ^.{4} to allow first five characters to be left unmasked.