Walking Poles-You will need these to get across rivers even if you do not like using them take them-strongly recommended

Boots- I would not use leather walking boots as they will not dry out on Stage 4 instead go for non-leather boots or trail shoes non leather

Invest in good waterproofs-strongly recommended

Lightweight tent if you can afford and lightweight backpack

Waterproof matches or cigarette lighters for starting fires in Bothies

Stage 4 of the Scottish National Trail which is the Cape Wrath Trail. if you purchase one thing make sure its this book https://www.amazon.co.uk/Cape-Wrath-Trail-British-Distance/dp/1852848170/ref=sr_1_1?crid=171Q1L6O8OA2T&keywords=cape+wrath+trail&qid=1569825821&sprefix=cape+wrath+trail%2Caps%2C143&sr=8-1  strongly recommended

Good torch, whistle. Best time to walk the Scottish National Trail, April-End of September


Consider a satellite device if walking alone especially on Stage 4 if you are injured. Review here: https://hikingguy.com/hiking-gear/in-depth-garmin-inreach-mini-review/

Headgear or Hat

Compeed for blisters strongly recommended  https://www.compeed.co.uk/product/blister/

Ordance Survey Maps, the UK's mapping agency, my suggestion is to use the OS app with a subscription for on-line maps which can be downloaded for off line use. You can then cancel the subscription once you have completed the Trail. Here is a good review of your navigation options    https://www.alexroddie.com/2019/01/skills-guide-modernise-your-mountain-navigation.html