I have printed all odd # pages from a document, but my HP Laserjet 4 won't feed these pages back through to print on the blank side with even # pages. I've tried to keep the front (feeding edge) of papers from curling up, but there's just enough curl in the paper from the initial printing that keeps it from feeding in properly. Any ideas?

If the same fault occurs with different printers it is almost certainly caused by the document.
"...second envelope from same document" suggests you are using a mail-merger. Is that right?
What happens if you start again from scratch? Mke a new document and try to print it a a single entity, without using mail merge?

this unit prints one side of paper only
to get it to print 2 sides when you go print on a document thee will be a section that says print all pages , print this page ,print elected pages
in the print selected section type in the pages you want to print ie 1,3,5,7,9 or even pages 2,4,6,8
or there may be a section that opens up with print all pages, print odd pages ,print even pages
to print both sides select odd pages and when they have printed take the whole of the pages and reverse them in the printer feed and then select even pages and print
the operators manual will explain how to do ti.

What document you are trying to print?
If it is Excel or Word document you have created a second blank page, reopen the document and delete the second empty page.

HP P4515n is not a duplex printer by default. It is a network printer. ('n' suggests it is a network printer. Duplex printers have 'd' in the model number).
If you want to print both sides, manually feed sheet by sheet. Or else you can do the following:
If you are using, say MS Word, print only the odd pages (Print the normal way. No need to manually feed paper). Then arrange the printed papers in the proper order and keep the printed side as back side. Insert the set of papers in the printer and print only the even pages.
First print a document of about 6 pages to get the hang of it.

To automatically duplex, you need the optional duplexing mechanism.
If you don't have this, your only option is to print the odd pages first, put the papers back in the tray, then print the even pages.
When you put the pages back into the paper tray, make sure they are print side up.
Try with a 5 or 6 page document first to be sure you get the orientation right.

If you HAVE the optional duplexing unit (which sticks out the front just above the main paper tray) then post a reply here and I'll try to help you further.
99% of the people that own this unit do NOT have the optional duplexer!

This could be a setting on the printer, in the driver on your computer, or the document you are printing.

First, try printing a different document and see if the problem persists.

Reset the printer to factory default settings, also referred to as a cold reset. HP has instructions on their website here: HP LaserJet 2300 Printer Series Resetting the Printer Cold Reset

You can also go into the printer driver on your computer and reset settings there, or uninstall and reinstall the printer.

I don't see multiplexing as an option for your printer.

The manual way to do this though is to go through your regular print procedure but choose the option to print only the odd numbered pages and then simply put the printed pages back into the printer tray upside down (after giving them some time to dry) and then print only the even numbered pages.

When you call up your printer (usually [Ctl][P]) tell it to print even numbered pages only, but in reverse order, This means the last page to be printed will be page 2, and it will be face down on the top of the pile (blank side showing) Take the printed sheets and rotate them so the top (head) of the sheet is feeding back into the machine, with the blank sides up. Now call your print up again and tell it to print the odd pages only. The first sheet to be printed will be page 1 which will print on the blank side of page 2, and all the rest will follow in sequence, with the correct sides up. Works every time. The print control is part of the Windows system, so your application should provide the full functionality.

HP LaserJet 1022 Printing Blank Pages

There is no option in the printer to chose eve or odd pages bout if you print under windows environment you can do that from the able cation example if you use Microsoft world go to file chose printer then chose eve or odd from print comp box