Ratagan through Glen Affric? then east through the Monadliath & Cairngorms and south to Montrose? should be do-able - just depends on choice of route. Expect some walking tho'.
4 days? Maybe. Depending on weather and whether they carry camping gear or go B&B/5* Hotel.
Could drop down and go through the Corrieyairack depending on what they want to do.
WANTED: Kona Sex One rear brake canti hanger pivot thingy - GOT ONE!
I could be tempted by a 19" - 21" Dave Yates (mountain) frame.................and no I still don't do trail centreshttp://www.corrieyairack.org/index2.html