Are you responding to an out of date alert form Secunia PSI?
Secunia will flag msxml 4 it as out of date because it is now end of life. msxml4 will have no more updates, therefore (barring proof of a vulnerability) there is no way to know whether or not it is now a security issue - hence the Secunia warning..
Installing msxml6 will no upgrade msxml4 but will install alongside it. The one question not answered in that thread is what is likely to break on installing mxsml4. You may well have some applications dependant on it.
I reluctantly chose to whitelist msxml4 in Secunia after checking I was on the latest available version (4.30.2117.0)
Edited by x64, 12 August 2014 - 01:09 AM.