John Gill Commentary Mark 4:34

"and without a parable spake he not unto them: but privately to his own disciples he expounded all things." — Mark 4:34 (ASV)

But without a parable spoke he not to them
For the above reason, as well as for the accomplishment of Scripture; (See Gill on Matthew 13:34), (See Gill on Matthew 13:35).

And when they were alone, he expounded all things to his disciples :
after they returned with him from the sea side, to the house in Capernaum, where he usually was when there; see (Matthew 13:36) . The multitude being dismissed, he unfolded and explained all these parables to his disciples, and led them into a large knowledge of himself, and the mysteries of the kingdom of heaven; whereby they were furnished for the work he had called them to, and designed them for.