Can I purchase this in Switzerland in Geneva when I am there?
Yes, it is possible to get both Interrail and Swiss Pass (Swiss Travel System), but maybe it's better to take a look into
SBB SuperSaver tickets. Since if you plan to make only 4 journeys and flexible +/- hour or two, then we're pretty sure that discounted SuperSaver tickets can beat £130. For instance for next Monday there are plenty Geneva to Bern SuperSaver tickets for 29,4 CHF, Bern to Interlaken on Tuesday can be done for as low as 16,2 CHF, Interlaken to Luzern on Wednesday for 12,4 CHF, Luzern to Geneva is 45,5 CHF etc. So at the end you can easily manage all your journeys for like 100 CHF (or roughly 70 GBP)
It's too late to get one now in the UK as I travel on Monday.
In UK (at least in London for sure) it can also be bought at the counter, but again pass might not be the best option