If all Pentium CPUs (before the name Pentium is reused by Intel) are 1 generation, then you have to disregard the changes in architecture and die shrink in between, and by that convention, the Core "i something" will all be 3rd gen (Nehalem, Sandy and Haswell are all named Core "i something"). What about the ones before Pentium (e.g. 4004, 8080, 80386, etc)? So surely Pentium is not 1st gen.
Core series (yes, that should include the Core Duo, Core 2 Duo, etc before the Core i series), and 4th gen makes perfect sense.
1. Core (Conroe>Wolfdale/Kentsfield>Yorkfield)
2. Nehalem (Nehalem>Westmere)
3. Sandy Bridge (Sandy Bridge>Ivy Bridge)
4. Haswell (Haswell>Broadwell)
But well, now it is more common to start counting with Core "i something" and looks like:
1. Nehalem (Nehalem>Westmere)
2. Sandy Bridge
3. Ivy Bridge
To avoid unnecessary confusion, just stick with what people use and call it the 4th gen.