Help identifying unusual Technic part, long beam with 4 studs on end
That’s a crane-arm centre. It’s usually used in non-Technic sets together with the crane-arm outside to obtain a telescopic arm, e.g., for cranes or firefighter ladder trucks. The slit at the end is to insert a piece of string for using it as a cable crane.

Here's a picture showing this piece in use as the boom of a crane from 4552: