Hey,
Had a quick google so don't take this as fact but it looks like the main differences, aside from the connectivity, is the preamps and software architecture.
The latter may not mean a lot, or anything, but it may mean longer lasting OS support into the future.
The 2i4 has 'award winning preamps' whereas the Clarett has transformer based 'ISA inspired' preamps.
Focusrite do get a pretty good name for their upper end preamps so, by the sounds of it, the Clarett wants to punch into that turf.
Nothing will beat first hand review, though.
If the question was a simple all-else-equal is thunderbolt worth $300 more than USB, the answer would probably be no although, on a mac, firewire or thunderbolt is the logical choice.
It gets the worst name possible amongst PC users and the best from mac users. There's a reason for that distinction.
When shopping, keep in mind that firewire is directly compatible with thunderbolt.
It's starting to fall out of favour in general but if you can get a cheap compatible firewire interface and the connector adapter, you might save some $$$.
Just a thought.