Google throws up no results at all for any kind of IC with a model code MD4020 or MD 4020 - but you'll already have seen this from your own search attempts
There was a CMOS ripple counter with part number 4020, that was in a standard 0.1" pitch 14 lead DIL package.
1) What fault symptoms you are trying to cure
2) How you know the IC is a voltage regulator
3) Why you think it is at fault
4) Whether there are any other identifiers on the package.