I mean I dived into the sea up to 4 metres under the surface of the sea.You can't dive up in water.
Do you mean
I dived down four meters
I dived (up to 4) meters - some amount less than or equal to 4 meters.
Underwater is probably not needed, but redundancy is not always bad. You've probably already told us you are at the beach, the pool, etc. Scuba tanks also makes it clear (the 'u' in "scuba" is for "underwater").
The verb is "to dive".I'm wondering about the use of the verb "to dive to a certain depth"