A lot of previous X fans, expecially X3 AP were extremely disapointed by Rebirth and somewhat by X4. This is not the same games, and if you were used to X3 there is a chance you might not like X4.
X4 is a mix of all the good things they could scrapped from Rebirth (which looked more like X1 and a fps space game) and the old features people wanted to have from X3. So this is the reason the reviews are mixed. It is hard to please everyone.
TBH I just want Privateer with the possibility of owning and building a larger working company like in Euro Truck Sim... This was not really possible in X1 to the extent I wanted it to, although it was the best space shooter of them all, with a proper story, a working living universe, conflicts and reputations and all.
X3 had the company/managment part down, but it's universe never ever ever ever worked, no matter how much you patched, modded, cheated. It always crawled to dead stop within a couple of weeks of playtime, and it was a god awful space-flight/fight sim.
So I'm looking forward to X4 being patched enough and working well enough combining the two. But I just spent that money on getting all the map DLC for ETS2, so I'll be busy with taht for a while.