How 'granular' do you want to go?
For instance:
Very early production M4s, M4A1s and M4A2s had M3 bogies and a bolted transmission housing. The M3 bogies were quickly superseded by M4 bogies (of which there was an early and late version). The bolted transmission cover was replaced soon after (again there was an early, rounded version and a later 'sharper' version). All M4A3s had a cast transmission housing and M4 bogies. All M4A4s had M4 bogies and a bolted transmission housing).
All early production M4s, M4A1s, M4A2s, M4A3s and M4A4s also had direct vision ports for the driver and co-driver and the narrow mantlet. The direct vision ports and narrow mantlet were replaced on all marks by periscopes, resulting in better armoured 'hoods' for the driver and co-driver. The narrow mantlet was also replaced by the wider M34A1 mantlet in late 1942.
It gets more complicated as the number of variants proliferates. For instance, the pistol port in the turret side was eliminated in 1943 then reintroduced in 1944. Appliqué armour was added, both during production and in field workshops (a lot of upgrading was done in the UK ahead of D-Day). It wasn't uniformly fitted to all marks.
And so on...