I take it you've updated the BIOS on your laptop that won't boot via USB to the latest version? As you know, BIOS updates can provide both fixes to existing problems and new functionality (such as the ability to boot from USB). I had a non-USB-bootable laptop circa 1999 that had this very problem until I updated its firmware. Check the manufacturer's website for your system.
If the above doesn't help, and you're positive the BIOS doesn't contain any option to boot from USB, you're next best bet is to create either a bootable CD or floppy disk that contains the USB drivers for your system, which you can use in conjunction with your BT4 USB stick. Have a search around the net and you'll find the details on how to do this.
Regardless, investment in a larger USB disk is your best bet - they're getting quite cheap now (as low as $10 for an 8GB) and will negate the requirement for multiple media types.