After complaining in an old post how the ferry timetable is a convoluted maze, when the 2026 schedule came out yesterday, I looked at it to see if there were changes and I discovered a mistake in my previous work. There IS the possibility of visiting all four islands with a single night on each without leaving the area. Saturday morning from Mallaig to Rum, then Sunday Rum to Canna, Monday Canna to Eigg, then Tuesday to Muck and Wednesday back to Arisaig. Only the initial 7:30am ferry is not at a nice mid-day time, giving two half-days per island. There's an easy bus from Arisaig back to Mallaig if you need to retrieve a car.
Too many places and too little time...