A four character limit gives you 10000 devices using only numbers. Is that really enough? I guess you could always use the complete alphabet like Dell does for their serial numbers.
All newer devices contain a asset tag field in the BIOS which is seperate to the serial number & you can update that. From there you can easily create a custom inventory for that BIOS item.
Both Dell & HP have client connectors for Altiris that you can leverage.
As to entering the string into the BIOS for the first time, hmm? You don't want ot do this by hand for too many machines. Maybe you can create a CSV file with a format of
"PC serial #","asset tag"
and then have a script do a lookup and use the correct value when updating.
Would be interested in what your final solution is.