A.T. Robertson Commentary Mark 4:10

"And when he was alone, they that were about him with the twelve asked of him the parables." — Mark 4:10 (ASV)

When he was alone (οτε εγενετο κατα μονας). Only in Mark. Vivid recollection of Peter. Mark has also "they that were about him with the twelve" (ο περ αυτον συν τοις δωδεκα), Matthew and Luke simply "the disciples." They did not want the multitude to see that they did not understand the teaching of Jesus.