John Gill Commentary Mark 9:4

"And there appeared unto them Elijah with Moses: and they were talking with Jesus." — Mark 9:4 (ASV)

And there appeared unto them Elias with Moses
Or Moses and Elias, as all the Oriental versions read, as in Mt. 17:3, (See Gill on Matthew 17:3):

and they were talking with Jesus ;
concerning his decease, and what he was to do and suffer at Jerusalem, and of which he himself had lately talked with his disciples; so that this might have been a confirmation of these things to them; (See Gill on Matthew 17:3).