An authorized agent in either of those places can customize and make you a private tour. You need to start a dialogue with an agent as to content and pricing. Bhutan is pretty much $200 or $250 USD per day, all-inclusive. Tibet will vary but figure at least equally expensive, but with only 4 days, you wouldn't have time to do much more than Lhasa and anything daytrippable from there due to distances involved. Tibet requires a Chinese visa and a Tibet Travel Permit that the agent will procure for you. If you will be accessing Tibet from Kathmandu, you need a special form of the Chinese visa obtainable only there. Tibet is generally closed to foreigners from around mid-February through end of March, and may be subject to spot-closures at other times.

IMO, it's probably not worth the effort/expense to go to either for such a small number of days.