I suspect you'd get a very different list, in terms of players, depending on whom you ask. A non-player would likely reference more populist musicians like the Beatles; these days, does anyone other than guitar players really even reference Van Halen at all? For me, I'm not honestly sure which four I'd narrow it down to, but I would certainly say that there are a lot of big names in guitar world that I wouldn't pick if it were up to me to choose those who were most important to *me*.
Hendrix, for sure. Johnny Ramone. Chuck Berry, and maybe..... Joe Moretti? or Link Wray.... I'm sure amore common version would have Clapton, Harrison and Page, or maybe Beck.
As to the men behind the guitar as a physical instrument as well as those who defined how we play it........ sticking to the electric guitar cause it's just easier, for me the giants would be Lester Polfus, whose ideas on the solid body concept influenced Leo Fender, whose guitars and amps were played by Buddy Holly who took what had been viewed as a country guitar into rock and roll (as well as establishing the two guitars, bass and drums lineup), and Jimi Hendrix, who reinterpreted how it is played to the extent that, now closing in on fifty years after his death, he remains adirect and primary influence on current players to an unmatched degree. Maybe that's my four?