The Montenegro trade has been quite well-known now for a couple of years. Property prices might very well increase... but this is large dependent on regulatory stability. Just vacationers won't pump the price enough. You need people moving there and living there and an absence of property taxes. They will likely build way more there over the coming years, with not a lot of regulations holding them back. The political environment itself is definitely very risky. A proven pirate nest is more attractive in my opinion. For what you want, I would say Malta is good. Yes, very overpriced, but the country is set-up in a way that will make it close to impossible for prices to go down given the massive attraction it has become to people from SE Asia / India / Serbia / Georgia / Ukraine / Bulgaria due to their insanely lax immigration "laws" (there are laws, but there is zero enforcement).
I would personally not see Montenegro become an immigration hub ever, it's simply too weird of a nation for that. A risky play for 4% returns, that's for sure, unless you want to go there on vacation yourself.