Is it possible to film in 4K with 120fps?

No, in 4K the maximum frame rate is 60FPS; in HD, recording up to 120FPS is possible.

These are the supported Slow & Quick recording modes:

Slow & Quick Motion

XAVC Long, XAVC Intra:

• 2160P: 1 to 60 frames

• 1080P: 1 to 60, 100, 120 frames

• 720P: 1 to 60 frames

MPEG HD 422:

• 1080P: 1 to 60 frames

• 720P: 1 to 60 frames