The obvious difference is a Euro 4 M3W has extra cats in the exhausts. There are others but they may have been changed by the owner or before delivery, air intake, ride height for example.

See also other details in Andys buying guide https://www.mtwc.co.uk/wp-content/uploads/2020/07/Final-150.pdf

and this thread

https://www.talkmorgan.com/ubbthreads.php/topics/480834/morgan-three-wheeler-euro-4-changes

2021 M3W
2013 M3W - Sold
1971 4/4 2 Seater
1934 Sports