Hi Bob,
That's correct. Day four is not a travelling day, so if you want to opt out of the walking excursion then you're more than welcome to. The walking group will be having lunch whilst out on the excursion, if you do chose to opt out, you will miss out on this lunch unfortunately.
Regarding suggesting a walk, unfortunately the TM or the local guide will not be able to help due to health & safety reasons. If you're feeling adventurous though, this would be perfect timing for you to explore the area individually.
I hope this helps.
Kindest regards,
Monty