To answer some of the above (I hope):
X4 allows you to progress from lonely ship commander to owner of a corporate empire with bases & the works. It looks more like a business sim in places than a space sim with a lot of action taking place via menus. It is single player with a pseudo-MP mode being added which will enable NPCs from different players' games to meet but not actual players.
Elite Dangerous is much more immersive & more about the commander and their ship. It suffers from grind in some areas but a lot of this problem has been removed with a reworking of the engineering, exploration & mining systems to remove the games addiction to RNG. It is MP & has landing on non-atmospheric planets unlike X4. The base game & Horizons seasons pass gets you Elite Dangerous Seasons 1 - 3, ie Base Game, Horizons & Beyond. Season 4, 'New Era' will be the next paid for expansion.
In a nutshell, get X4 if you want a game that will take you on a journey from small fry to big fish in an isolated world. Get Elite Dangerous if you want to fly a space ship, pretend to be Captain Picard to a growing degree & maybe find other people to do it with. X4 is a largely complete game, Elite Dangerous isn't remotely complete and is aiming to be where Star Citizen is heading but is arguably a functioning product & completely playable product whilst Star Citizen is still very much a tech demo.
Hope that helps