You're really lucky you didn't destroy the SCSI controller in your Mac by hooking that parallel printer to it. People have done exactly that by making the mistake of plugging one of the parallel port ZIP drives into said port. Hopefully the printer is okay.
(General warning about dealing with old/obsolete technology: unless you know for sure do *not* assume that merely because the plugs on two different pieces of equipment are the same that it's okay to plug them together. DB9/15/25 plugs were used for *all sorts of things* back in the day and cross-connecting some of these things will let the magic smoke out.)
And yes, to be useful to a "classic" Mac a laser printer pretty much has to have Postscript capability and so far as I recall the 4 Plus doesn't have it. Likewise I'm pretty sure the Postscript-equipped versions had an "M" appended to their model number, so the PS equipped version would be the 4M+, and I also think, but wouldn't swear to it, that the M versions came equipped by default with a Localtalk interface card in addition to the parallel port.
It's possible to upgrade a plain 4+ to a 4M+ by swapping in the appropriate parts from the latter, but I suspect it'll be more than a minor pain to acquire the parts ala carte. How are all your systems networked together? (I assume they are?) If you're using Ethernet one low/no cost strategy if you have a dingy old PC with a parallel port sitting around would be to set up a Linux print server with Ghostscript (a Postscript emulator) and expose it to the Macs via Netatalk, which would allow them to use the 4+ you already have. Fair warning, this is one of the less easy parts of Netatalk, but once it's set up it works well.
(General warning about dealing with old/obsolete technology: unless you know for sure do *not* assume that merely because the plugs on two different pieces of equipment are the same that it's okay to plug them together. DB9/15/25 plugs were used for *all sorts of things* back in the day and cross-connecting some of these things will let the magic smoke out.)
And yes, to be useful to a "classic" Mac a laser printer pretty much has to have Postscript capability and so far as I recall the 4 Plus doesn't have it. Likewise I'm pretty sure the Postscript-equipped versions had an "M" appended to their model number, so the PS equipped version would be the 4M+, and I also think, but wouldn't swear to it, that the M versions came equipped by default with a Localtalk interface card in addition to the parallel port.
It's possible to upgrade a plain 4+ to a 4M+ by swapping in the appropriate parts from the latter, but I suspect it'll be more than a minor pain to acquire the parts ala carte. How are all your systems networked together? (I assume they are?) If you're using Ethernet one low/no cost strategy if you have a dingy old PC with a parallel port sitting around would be to set up a Linux print server with Ghostscript (a Postscript emulator) and expose it to the Macs via Netatalk, which would allow them to use the 4+ you already have. Fair warning, this is one of the less easy parts of Netatalk, but once it's set up it works well.