John Lee Hooker played electric at Newport Folk in 1963, performing "Boom, Boom" and a variation of "Green Onions," among other songs. As noted by others, Johnny Cash and the Staple Singers performed with electric guitars in 1964, as did the Swan Silvertones. Muddy Waters did not--he played only acoustic, subbing for Son House and in a duet with Otis Spann. In 1965, the Chambers Brothers, Lightnin' Hopkins, and the Butterfield Band all played electric before Dylan.
I have never before seen a suggestion that either Estes or Rachell used an electric guitar at Newport in 1964, and would love to see the evidence--I've heard their sets, and it sure doesn't sound like either of them is amplified, except with microphones.