More features. Suppose it depends on if you plan to daily drive it. Mk4 fine if you're mainly hodling and infrequently transacting on chain with those keys.

The user interface is just nicer and its got a lot of fun toys, but if you're just using it for long term storage then im agnostic as to which is better. Once youve got your shit stored away who cares what the signing device is so long as the key generation was originally sound, both are good for that.

Thanks @DudeChicken and @SuiGenerisJohn