A tenant painting the walls is generally considered acceptable and permission to do so can't generally be denied without a good reason (e.g. you have special walls that would be damaged by painting or something). Landscaping also really shouldn't be an issue, unless they've torn out actual existing landscaping work and replaced it with their own; tenants will naturally want to arrange their gardens as they see fit, especially if they aren't done up at all in the first place. Changing the white goods could be getting iffy, but if they've safely stored the original units to be put back when they leave and they didn't cause any damage in the process then there's really no cause to object. (If they did cause damage or they've disposed of your original items, that's not really acceptable, though...)
Adding an outdoor tap (knocking a hole in the wall in the process) and adding a stove (if they also put in a chimney/flue or made other structural changes to accommodate it) without permission is going a bit beyond the bounds of acceptable behaviour, and changing the locks without permission is definitely unacceptable (what if you had to access the property due to an emergency situation?).
All that said, none of that could possibly give them any sort of claim of "ownership" of your property. Hell, even if they knocked down the walls and did a complete down-to-the-studs renovation, that wouldn't mean they own the place. Whether they'll be problem tenants who will overhold at the end of their tenancy is another matter, though; certainly putting in a lot of work around the place could make some people feel they've a right to stay longer and could make them ornery if the landlord decides to end the tenancy, and by doing these things without consulting you and getting permission, they've definitely demonstrated that they don't have much respect for proper boundaries in the tenant-landlord relationship, so that is certainly cause for concern.
As they've only been in there four months, they do not yet have Part 4 tenancy rights. Are they on a fixed term lease? If not, you can give them notice at any time before they've been there for six months for no reason at all; just be sure to do this quickly before they do hit that six-month mark. If they are under a fixed term lease, is there a clause in there that states the tenant cannot make changes to the property without the landlord's approval? Any proper lease should have a term to that effect. If so, then they have likely violated the terms of the lease and that should give you cause to terminate it. If there is no such clause, I'm afraid you might be a bit stuck, though; you can't end a fixed term lease early unless the tenant has violated it, and after six months they will acquire security of tenure for six years and cannot be removed even after the fixed term lease expires, except for an allowable reason (you require the property for yourself or an immediate family member, or you are selling it or performing extensive renovations that require vacant possession).