FreeBSD 2.1.7 was the last version of FreeBSD that could be installed on a 4MB system. FreeBSD 2.2 and later needs at least 5MB to install on a new system.
All versions of FreeBSD will run in 4MB of RAM, they just cannot run the installation program in 4MB. You can add extra memory for the install process, if you like, and then after the system is up and running, go back to 4MB. Or you could swap your disk into a system which has >4MB, install onto the disk and then swap it back.
FreeBSD 2.1.7 will not install with 640 kB base + 3 MB extended memory. If your motherboard can remap some of the ``lost'' memory out of the 640kB to 1MB region, then you may still be able to get FreeBSD 2.1.7 up. Try to go into your BIOS setup and look for a ``remap'' option. Enable it. You may also have to disable ROM shadowing. It may be easier to get 4 more MB just for the install. Build a custom kernel with only the options you need and then remove the 4MB out. You can also install 2.0.5 and then upgrade your system to 2.1.7 with the ``upgrade'' option of the 2.1.7 installation program.
After the installation, if you build a custom kernel, it will run in 4 MB. Someone has even successfully booted with 2 MB, although the system was almost unusable.