4 companies come into existence because historically the first US cell phone band was given in two parts, one to the local land line monopoly, one to someone else. All these were in small blocks of geographic area. The winners of this frequency started building cell towers and selling phones and quickly discovered their customers didn't like roaming changes. As a result they started consolidating: lots of little merges and buy outs. The land lines mostly became Verizon over the years as land line companies realized that running a cell phone network and land line are different enough that they couldn't do each effectively and so they spun the business off. In the mean time another block of frequencies was opened up and the result was 4 more companies got spectrum in each area (not the same areas of the first), and they too started merging. The big 4 are big because they have merged and/or bought enough spectrum to be essentially nationwide. There are some smaller regional companies left that are not nationwide.