A couple of things to consider. Since you have an IP card and 4 i2004 sets, you likely have the IP Telephony starter kit that was offered free with upgrades to 25.XX (X11 release 25.XX) software (I the program was called IPeXPRESS). That starter kit gave you 4 IP station licenses. If you're still on release 25.XX, you probably still have 4 licenses.
Starting with the release of Succession 3.0 (X21 release 3.0), the licensing is different, and the licenses (ISMs) for digital, analog, and IP stations come in increments of 8. When we did the upgrade from X11 25.30 to X21 3.0, we gained 4 IP phone ISMs because the minimum increment is 8.
One other consideration is that later this year (4th Quarter) Nortel will issue X21 release 5.0, which will move X11 release 25.XX to Retired status, and you will not be able to add IP phone licenses without upgrading to the new release. X21 release 5.0 will require an upgrade to the Pentium 4 processors in the Option 61C and 81C, which will require the Fiber Network Fabric in the 81C, so depending on the size of your system, at some point just adding a few IP phones could force you into a series of software and hardware upgrades that could run into a lot of money.
I would recommend that you consult with your distributor ASAP to map out your options and a strategy to manage the upgrades.