Depending on where your Mini was stored (humid or dry) and what if anything you did to the engine before leaving it, It may be OK.
If the engine oil was relatively fresh, it should be OK to test crank it, starting with a manual turn over to make sure it isn't seized. Squirt a bit (a tablespoon or so) of oil in each spark pug hole to wet the cylinders before you hand turn the engine. If successful, try the starter - still with the plugs out. This will help circulate  oil and bring the pressure up.  If all goes well, put the plugs back and see if it fires up. Proceed with head work if symptoms suggest.

Great news. I am looking to start my minis engine which has sat unmoved for 4 years... (I was far away!)

Great!
Thanks for providing an update.

Hey Gang,

Thank you all for the tips. I took the head off and that was a great idea. Cleaned everything up , lapped the valves and put it all back together. She turns over nice! 

Thanks again. 

Make sure you prime the oil pump and that you have oil pressure before starting the first time.  Info on how to do this is on previous threads.

Looked it up last night. Yes, I will do this for sure !!! Thanks for the suggestion.

Consider, while the head is off, to lap in the valves to clean them up. Easy enough to do. And your local autozone or Oreily auto parts have free loaner tools (valve spring compressor and lapping stick) to help you get it done for super cheap.

Thanks Guys.
I haven't cut any corners yet in this build......so.....Gasket Set on order.........that little engine that could is worth an afternoon of scrubbing. It was running strong when it last ran.  

2 trains of thought. A lot depends on what your plans are for it, the value of the engine, what condition it was in prior to it sitting for 25 years, where it has been stored and under what conditions, and your comfort/skill level as a mechanic. Oh, and how fat your wallet is. That's always a factor in everything.

1 school of thought:  What you are proposing is just fine. You are using a reasonable about of caution to determine if the engine might be "good enough" to just run it as is, especially if it is a run-of-the-mill small bore or even if it is a 1275 motor. They are essentially tractor motors and will run and in a variety of surprisingly horrible conditions.

The 2nd school of thought is that gaskets are cheap compared to the cost of pistons and bearings and a head rebuild. Pulling the head off might cost you a few hours and a few gaskets, but might save you from the equivalent of running gravel and/or MarvelMysteryOil&&Rust paste up and down your bores between your pistons and cylinder walls, or busting a ring on a ridge of corrosion crud that could have been scoured out with a green strubby and some elbow grease.

You could try turning the engine by removing the spark plugs, putting the transmission in 4th and turning one hub while the other hub is held immobile. Note the differential will try to turn the farther hub in the opposite direction from the one you are trying to turn.

Spark plugs out,
gearbox in neutral,

...Radiator in our out?

If radiator in, install fan belt and push/pull the engine over in the normal of direction using the belt.
If radiator out, large socket on crank pulley so you can turn the engine over with a breaker bar.

Alternate:  plugs out, gearbox in 4th gear, gently roll the car a short distance back and forth.

Are you planning to pull the head and inspect the bores?

Hello,
I am looking for advise on 2 things. My engine has sat for 25 years. I want to manually turn my engine over, very slowly to get it to make a full rotation and then I want to stop it at TDC.  I have poured 2-3 ounces Marvel Mystery Oil into the cylinders and let it sit for the past year. I have also ran 5 quarts thru the engine from top to bottom a few times in the past month.  So, now I am ready as I'll ever be. I do not want to turn it over with the starter.
So gang, any suggestions? 
Thanks in advance.