400 RPM is not rotating slowly and it does sound a little slow for the coarse pitch stops (min governed position), which is where a windmilling prop will settle to.
However at least one type of turboprop has an autofeather system which actually lets the prop cycle and this allows less drag than the feathered prop.
Any pitch lock system would be worse as it would lock the prop most likely in a finer than ideal position.
I have never experienced a prop ceased in fine pitch, but FI on either type presents a huge amount of drag, I believe the engine is not driving the propeller in either case, so what is ?.
Unless you have shut down the gas generator then it will still be producing some thrust however small.