According to Intel, the 946GZ chipset supports the following processors:

Click on compatible products on the left.

Now, just because the chipset supports it doesn't mean Lenovo ever did BIOS compatibility, BUT, I did find an obscure little form:

It shows ordering info for both P4s and C2Ds, so likely any processor listed at ark will work. You can try Lenovo support via phone and see if you get someone who knows CPU support for the BIOSes they released. Good luck.