#x4

What does "x4" mean?
xf100_400mm, xj_4x4, x_4

x4-l 8249 x4-5 2437 x4-0 1900 x4-m 460 x4-2pspmh 1668 x4-3pspmh 1657 x4-2smpmh 1574 x4-3smpmh 1581 x4-7pspmh 1320 x4-7smpmh 1251 x4-sj 1190 x4-koudai 909 x4-anewu 737 x4-shota 1112 x4-1pspmh 603 x4-yuya 3146 x4-kodai 1792 x4-jukiya 854 x4-5challenge2014 423 x4-8pspmh 451 x4-8smpmh 425 x4-tour 314 x4-5challenge2015 341 x4-4pspmh 300 x4-9pspmh 285 x4-1smpmh 276 x4-vv 574 x4-gravaçõesestouradoo 207 x4-sjsj 151 x4-tunedotcom 168 x4-tmax 2037 x4-4smpmh 148 x4-9smpmh 140 x4-gconcert 124 x4-fun 118 x4-m40i 324 x4-7 475 x4-0fin 500 x4-r 425 x4-rsb 142