You could look up on
www.sitefinder.ofcom.org.uk to see who the mast belongs to and its registered output power, from there you can get a more accurate idea of the coverage. But the most reliable way is to just get a free PAYG SIM off 3 and put it in your phone. As a hark back to the topic itself, they'll send you a free SIM with no tie-in and no commitments complete with all the "optional" crap BT charge extra for.
As for 4G, as I've mentioned EE plan to provide at least 99.2% coverage with "superfast" 3G and 4G has considerably more range (if done right, but only Vodafone and O2 are in a position to provide decent service over it, yet both companies are currently taking the "lazy" approach) so having it provide broadband to remote notspots is quite economical. If 3 put in coverage there (which I suspect they will, if they've just put a new mast in recently) then it's likely it'll get 4G, and on an unlimited package with line of sight you can expect ~100Mbps which could then be shared among a few locals.
Even without the government's involvement, you're likely to get 99% coverage anyway - as I say commercial deployments are targeting 98-99% coverage by population, and unless there's a disproportionate excess of single person homes in rural Scotland that'll likely mean a similar number of properties.
Mind you, now that I think about it if that new mast is a 3 mast it should also give you Orange/T-Mobile 3G signal too, as they all share the same 3G network.