It sounds like what you've got is a "Type B" voucher. You should have 5 or 6 pieces of paper - one covering terms and conditions, (possibly) one with a summary, and then 4 vouchers, each of which is for $100 giving a total of $400. On each of the vouchers it should state "Treat as Type B Voucher".
"Type B" vouchers are basically able to be used as payment for any United/United Express/Continental flight. This is different to many other United vouchers (aka "E-Certificates") which give a "discount" on the fare, rather than being treated as payment for the normal fare.
You can use one or more of the $100 vouchers on a single booking, although you will not receive any credit for using only part of a voucher. eg, if your flight cost is $280, you can give them 2x $100 vouchers and pay $80, or you can give them 3x $100 vouchers but you will lose the additional $20.
Unfortunately you can't book flights with "Type B" vouchers online - you need to go via the United call center. If you're a Mileage Plus member you can reach them at the number on your card (which may take you to a dedicated reservations center, depending on your status). If you're not a MP member, you can call 1-800-UNITED-1 (aka 1-800-864-8331).
Note that there is NO charge for a phone booking when using a voucher! Normally you would be charged to book over the phone, but they will waive this when booking with a voucher. Depending on the agent you get it's possibly they will try and charge you a fee - if they do, just remind them that you are using a "paper voucher" and they will remove the fee.
Once you've booked your ticket the booking will be placed on hold until you pay for it by getting the voucher(s) to United. You can do this by either mailing the vouchers to United (I'd suggest registered mail, just in case!) or you can take the vouchers to a United ticket counter at any airport.
Personally I'd suggest going to the airport - it may cost you a little more, but it gives you the piece of mind of knowing that the vouchers aren't going to get lost. If your flights are less than 2 weeks away this is the only option they will offer you.