If it's the HP LaserJet 4P you're referring to (and not one of the newer 4000, 4050, or 4100 series) then it's an older model. Take a look here to start:
I believe the second URL is more accurate as far as per-page speed is concerned. If I remember correctly, the LaserJet 4P had a 16MHz Intel i960 RISC processor, which is actually a bit slower than the 4Plus (25 MHz) or even the plain 4 (20 MHz).
I own an HP LaserJet 4 Plus, and a plain HP LaserJet 4. They are fantastic printers and were certainly not a waste of money. I bought them both for $25 (total for the pair) from a dot.com blowout auction. I knew they would need some care and attention. I spent about another $350 (again, total for the pair) refurbishing them, and that was to replace everything that was even remotely worn on them. Does that make them a good deal? Probably not to some, but for me it was a thrill to get these beauties and to put them back into top-notch shape. With the possible exception of the L's, V's, and P's (sorry!), the LaserJet 4 series printers are absolute workhorses. They are also so common that HP still sells a fair range of replacement parts (check out http://partsurfer.hp.com/ ) and the toner cartridges are cheap (~$50 to ~$70 and last for thousands of pages) and ubiquitous (even warehouse/club stores often have series 4 cartridges). In their now-refurbished condition, I expect both of these printers to last for at least another 100,000 pages.
This page should help you get a rough age of the printers in the 4 series:
The other thing you'll want to do is run a Self Test from the control panel of the printer. Aside from checking its tiny brain a bit, this will print out 2 important things about the printer in question:
1) the total number of pages the printer has printed in its lifetime
2) how much RAM is installed
It'll also tell you if an optional PostScript memory module has been installed, or if the printer just has the default PCL capability. (PCL suits me just fine, but some people need/like PostScript.)
With that knowledge, here's a useful point: From those earlier links, you know that the 4P came with 2 MB of RAM installed at the factory. The printer is also rated for 600 DPI max resolution. However, if the 4P is like the others in the LaserJet 4 series, you'll need more than the original 2 MB RAM installed to take full advantage of that maximum resolution. I don't have information about the 4P or others in the series, but here's the deal for my HP LaserJet 4:
OPTIONAL MEMORY REQUIREMENTS:
Full-page (letter-sized) 150-dpi graphics None
Half-page (letter-sized) 300-dpi graphics None
Full-page (letter-sized) 300-dpi graphics None
Full-page (legal-sized) 300-dpi graphics None
Half-page (letter-sized) 600-dpi graphics See Note Below
Full-page (letter-sized) 600-dpi graphics See Note Below
Full-page (legal-sized) 600-dpi graphics See Note Below
The optional memory required for the HP
LaserJet printers is directly influenced
by the complexity of the graphics and
soft fonts being used. SIMM
specifications for the HP LaserJet 4
printer are 72-pin, 36-bit, and 80 nS.
Presence detect must also be available
on the SIMM.
This was one of the sneaky things HP did back when it was a big deal that any printer could do "high" resolution graphics (i.e. they could get away with this back then). What this means is, if you want to be guaranteed the ability to print a half page or more of 600 DPI graphics, the memory that comes in the printer by default is not sufficient. You have to upgrade the RAM. So yes ... check to see how much memory is installed in the printer. If it's only the default amount, you probably won't be able to get the maximum resolution out of it - at least not with complex graphics. If you do wind up buying a LaserJet 4 of some type, and need some compatible RAM, let me know. I've still got some 4 MB SIMMs that will work in the 4 series that I could sell. It's hit or miss as to whether any old type of SIMM will work or not because it needs to be A) parity RAM, and support the evil "presence detect" function.
Now that I've paused writing I'm not sure if anything I've said here is helpful or not. ;-) I mentioned that I don't think the 4P is quite as good as some of the other models in the 4 series, but other factors come into play there - like how old the printer is, how many pages it's printed in its lifetime, whether it has extra RAM, etc. Tough call really. If it were a plain 4, 4 Plus, 4MP, or 4Si, I'd say there's a strong reason to consider taking it. For a 4P, I'd really look strongly at how the price works in with the age, page count, RAM, whether it still has manuals, etc..
In case it's helpful, to understand some of the variant model names for the HP LaserJet 6's and earlier, here are some personal notes I came up with (very unofficial):
A "P" in the name means "Personal" use (i.e. not industrial use).
A "V" in the name means "Value" (i.e. not heavy use).
An "M" in the name means "Macintosh" (i.e. it's got PostScript capability).
An "L" in the name means ... well ... "Lemon" (eeww!).
An "N" in the name means "Network" (i.e. has networking capability by default).
An "Si" in the name means "Seriously Intense" (i.e. you could print enough paper to wallpaper a small town with this guy)
The above descriptions I came up with for model numbers only work for the HP LaserJet Series 4, 5, and 6. They don't apply to the 2xxx, 4xxx, 5xxx Series, etc. or others.
Final thought (I swear): Do NOT consider any HP LaserJet III or earlier. They are too old and slow (and heavy!) to be worth the trouble.