Agree that 4 days is not enough.
I've taken the St John to Digby ferry and it would be good if you intended on exploring the Bay of Fundy side of Nova Scotia. If you want to hit the Cabot trail, I think the ride up is more direct and has less waisted time waiting on Ferries. The ferry would only move you east, so all of the North travel would still need to be ridden.
I would agree that 4 days ins't enough time to visit the Cabot Trail. I think you would need at least 5, since two travel days are needed each direction.
The Hopewell Rocks area is really nice, and that could be done in your four day window.
I just returned from there last weekend and it was a great visit. Consider going up via the Calais crossing but returning using Dear Island ferry to Campobello Island. It's a lot nicer, the first ferry is free and the second one is about $12. It's also nice to not have to retrace your route up. If it works out for you there is a campground on Dear Island where the Campobello ferry leaves from. I haven't stayed there, but Dear island is really photogenic.
DL650K6, MadStad adj shield, Givi e45 & e36, gpsmap 76CSx, Pat Walsh skid plate & bars, Sargent seat
[URL="http://www.sr.unh.edu/~rea/stromtrooper/2007/Strom_Range.png"]My 2007 Range map[/URL]