I have an HP LaserJet4 and I'm suddenly (as of last evening) getting a 41.3 error code followed by a 13 Paper Jam. I clear the jam, warming up, blank page is ejected, I put the printer back on line, select next print job, first page fine, second page fine, 41.3, then paper jam. What's going on?

41.3 - points to a miss match in the paper tray.
First -
Pull out your paper trays, take the paper out and confirm the guide in the back is set to the correct position. (The little mechanism on the tray which allows you to use different paper types, example letter to A4).

Second -
The other which may have happened it the paper handling was mis-configured on the machine:
- Press the Menu Button on the machine.
- Using the + and - button go to the Printing Menu.
- Select Paper confirm that the paper size is correct for the trays within the machine.
Third -
- Depending on the document you are attempting to print make sure the document type is either adjusted to the type of paper your using or to load the correct paper into the machine. I seen where people send you a legal document and your attempting to print it on letter.
Fourth -
It could be a driver related issue, 98% of the time this is not the case. If all fails above, then attempt this.
Let me know if this helps!
Lastly it could be a driver related issue, however if more than 1 person uses it don't count on it.