I don't believe that the GeForce4 architecture can, in any way, support PS1.4. If it could, you had better believe that not only would they support it, but the added functionality would be exposed in OpenGL (In particular, GF4 architecture can only support up to four textures per pass, and in a less flexible fashion than is available in PS 1.4).
Regardless, the reality of the situation appears to be this:
1. PS 1.4 offers one additional real-world improvement: overbright lights. The DOOM3 engine supports these, but apparently the designers aren't taking advantage of them (JC is attempting to push the designers to go for it...). While the overbright lights will be visible on all hardware, there will likely be visible banding on DX7/DX8 hardware. What PS 1.4 offers is much better accuracy under these situations.
2. PS 1.4's larger texture addressing flexibility and larger number of textures allow the same effects to be done in fewer passes. This is the primary situation where we'll see PS 1.4 used. What this translatest to is that PS 1.4 will be used to make the same effects run faster on a Radeon 8500. However, in the real world, it still cannot outpace a GeForce4 Ti in these situations (i.e. the benefit is nowhere near as great as you'd think given the use of only one pass, as opposed to the 2-3 used on the GF3/4 hardware).
What this appears to translate to is that there is really little reason to go for PS 1.4. The performance alone will speak of 99% of the improvement that you'll see from it, and current data indicates that it won't help the Radeon 8500 out that much. Secondly, the only practical visual improvement that we've seen come forward is a big "if" that may never be realized.