Don't know how to operate LaserJet 4 Plus. - HP LaserJet 4 plus Printer

For which operating system. Any version of Windows starting with XP should have a native driver for this printer already installed.

Make sure that if you have a setting for paper type you have it set for envelope. This would be found in the setup menus, I just don't remember if this model has that setting, they are getting to be rare now. The other common cause of print problems is the envelopes themselves, if they are old or have been stored where they have been allowed to absorb moisture, you will have a very hard time printing on them. Try some different envelopes if you have them.

You can download the drivers directly from HP HERE. Just select your operating system (vista 32bit or vista 64bit)

Access the manufacturer's web-site: http://support.hp.com
and do a search for your specific model.
Select your Operating System.
Download the device-drivers.
Install them.

My suspiction is there is a tab on the tray that triggers a switch telling the printer what type of tray has been inserted. Your old tray is likely different from the Plus 4 tray and the printer knows it.

Scroll down to the I/O menus.

http://reviews.cnet.com/laser-printers/hp-laserjet-4-plus/4505-3159_7-30088336.html.3

printer keeps running until paper tray empty, msg 41.3

You could try ths site
http://www.eserviceinfo.com/

You can try to do a "Cold reset"

turn off the printer hold down  the Menu/reset button and turn it back on.

Do you hear the machine go into rotation? the 4 plus has a high pitch sound follwed by the fuser turning ( you'll know when it does it) if it doesn't go into rotation then there is another problem.