Yep, it's a stupid nube question.
It's also a question you should be able to answer using the search feature. Have you really been here for two adn a half years without stumbling across any of this?
what is the difference between length of the M4 front end, the "mid-length" front end, and the carbine front end?
Carbine and midlength refer to the length of the gas system. M4 is a specific length and contour of barrel that uses a carbine length gas system. Mid length is 2" longer than a carbine length gas system and 3" shorter than rifle length gas system.
Is the front sight base at the same position for each length?
The front sight is usually one piece with the gas block, so obviously the front sight is 2" farther forward on a mid length than on a carbine. There are aftermarket front sights that are not a part of the gas block and these could be at different positions.
What about handguard size differences?
Since normal handguards are held in place at the front by the has block, the midlength handguards obviously have to be 2" longer than carbine. M4 handguards are larger around than carbine handguards, but M4 and carbine barrels and handguards can be swapped around at will.