Jamieson, Fausset and Brown on Jeremiah 35:4

4. man of God--a prophet (De 33:1; 1Sa 2:27; 1Ki 12:22; 2Ki 4:7), also "a servant of God" in general (1Ti 6:11), one not his own, but God's; one who has parted with all right in himself to give himself wholly to God (2Ti 3:17). He was so reverenced that none would call in question what was transacted in his chamber.

keeper of the door--Hebrew, "of the vessel." Probably the office meant is that of the priest who kept in charge the capitation money paid for the use of the temple and the votive offerings, such as silver vessels, &c. There were seven such keepers [Grotius]. Compare 2Ki 12:9; 25:18; 1Ch 9:18, 19, which support English Version.

I said ... Drink--Jeremiah does not say, The Lord saith, Drink: for then they would have been bound to obey. Contrast the case in 1Ki 13:7-26.