Alright, lets make a list then
Basing on steamspy data we have:
Sid Meier's Civilization V 8,724,242 ± 72,442 owners
Sid Meier's Civilization: Beyond Earth 1,342,104 ± 28,767 owners
Sins of a Solar Empire: Rebellion 794,588 ± 22,155 owners
Stellaris 354,141 ±14,801 owners so far
Endless Legend 749,326 ± 21,516 owners
Endless Space 965,737 ± 24,418 owners
Galactic Civilizations III 231,336 ± 11,965 owners
Master of Orion 72,678 ± 6,708 owners
Anno 2070 815,515 ± 22,444 owners
Age of Wonders III 455,696 ± 16,787 owners
Polaris Sector 8,922 ± 2,350 owners
Total War: Shogun 2 2,792,575 ± 41,396 owners
Total War: Attila 717,368 ± 21,053 owners
New notable 4X titles coming this year (2016):
Stellaris (just released not so long ago)
Total War: Warhammer
Civilization IV
Endless Legend 2
Master of Orion: Conquer the Stars