In reply to waterfall:
Four days is a decent challenge walk sort of target. The route is waymarked throughout so you're not going to get lost. The Harvey Map dedicated to the WHW is worth having though.
I don't think you strictly need a guidebook, but these staged route descriptions may be helpful. They start/stop at obvious points, where there's accommodation (and food shops in a few cases); you'll need to run some together if you're aiming to complete the whole thing in 25-mile-ish chunks
Milngavie - Drymen https://www.ukhillwalking.com/logbook/r/?i=92
Drymen - Rowardennan https://www.ukhillwalking.com/logbook/r/?i=97
Rowardenan - Inverarnan https://www.ukhillwalking.com/logbook/r/?i=211
Inverarnan - Bridge of Orchy https://www.ukhillwalking.com/logbook/r/?i=282
B of O - Kingshouse https://www.ukhillwalking.com/logbook/r/?i=324
Kingshouse - Kinlochleven https://www.ukhillwalking.com/logbook/r/?i=25
Kinlochleven - Ft.William https://www.ukhillwalking.com/logbook/r/?i=526
Four days could go something like:
Milngavie - Rowardennan (groceries in Drymen en route)
Rowardennan - Crianlarich (accom and food shop)
Crianlarich - Kingshouse (passing a shop in Tyndrum on the way)
Kingshouse - Ft William (shop in Kinlochleven en route)
Discreet zero-impact 'wild' camping is possible most of the way, so you are not tied to the obvious stages listed above. Bring a water filter since you're in a very busy area and there's livestock (two sources of poo-related concerns).
Weather is anyone's guess, could be nice and warm and spring-like, could be more or less full winter, or perhaps most likely a wet and chilly halfway house. Check much nearer the time and be prepared for anything
Post edited at 10:16