4M4321 wouldn't be the actual serial number of the car. A Kaiser Virginian 4 door hardtop would be a 1949 model (4923) or a 1950 (5023)
From the Registry, here are two actual listings:
Serial No. Body No. PAINT TRIM DR TR AG SCH ITEM OWNER STATE
#23 K502-033310 1406 048 2306 1 4 3 103 186 Davis CA 1950
and
#27 K492--033621 1487 055 2314 1 4 3 130 292 Telatco OH LWR GMR
When they reserialized the 1949 cars as 1950 models, there were some cars that were out of sequence for serial number AND body number as this shows. (This also happened again with the 'other' Virginians and Vagabonds, which weren't hardtops or utility (hatchbacks) at all, but 1951 Kaisers and Henry J's that were reserialized as 1952 models, and sent out with a Continental kit (I think--there might be other differences) and are different from the 'real' 1952 models)
SN: Serial Number (on drivers door post)
BN: Body Number (on Firewall)
PNT: Paint Code
TRIM: Trim Code
DR: Delivery region (1 for US, 2 for export)
TR: Transmission code
AG: Accessory Group
SCH: SCHED(ule) Number
ITEM: Item Number
ADDL INFO:
HB, LWR, GGR, etc.: Numbers sent in by various members