HP? Donno. Start at www.hp.com.
FWIW, HP likely retired those printers many years ago; the follow-on LaserJet 4200 and LaserJet 4300 series I've been working with are probably seven or eight years old now, and your LaserJet 4000 models are yet older.
According to Wikipedia, the LaserJet 4000 and LaserJet 4050 series was discontinued in 1999. (When or if the support for that printer has ended, I don't know. That's a question for HP.)
If you're familiar with networking and have a wired network available (and most WiFi devices have that option available), then I'd get the JetDirect path going; that's vastly easier to manage and maintain than is dealing with any other path into those printers.
Assign the JetDirect a static IP address outside the pool of addresses the local DHCP server is using, and configure a network printer at that address.
On the LaserJet 4200n and 4300n series, those provide Bonjour announcements, which means they're really easy to configure; OS X finds those automatically. And those are web-managed, meaning you can connect to the printer via Safari, and check its status and supplies. (I don't know if the LaserJet 4000 series offers that; that's older than I've worked with recently.)
HP does (did) have a universal driver for OS X, though they've probably not modified the drivers for those printers in years. (A Google search for /HP LaserJet 4000 Series Full Solution Driver Mac/ does find a set of drivers of some ilk for OS X 10.6.)
The LaserJet 4000 is old enough that the generic Postscript driver in the Apple CUPS package in OS X will probably work fine, too.