This Answer Record covers the key points to consider regarding footprint compatibility between the 4K family and the Spartan family.
1) Spartan devices are footprint-compatible with other Spartan devices.
2) The positions of two "Don't Connect" pins on Spartan devices correspond to the M1 and M2 pin locations on XC4000
devices. These pins are reserved for future functions and must not be connected under any circumstances.
3) Spartan MODE pin cannot be used for I/O.
4) Spartan PQ208 and 4000-PQ208 have SIGNIFICANTLY different pinouts.