Question

What must you add if an introductory prepositional phrase has four or more words?
A. a space

B. a comma

C. a semicolon

D. a colon

Answer 1

C. Semicolon

Answer 2

B. a comma. A semicolon presupposes that the text beforehand was at least a complete thought and what is to follow would be about a closely related subject. Your "introductory prepositional phrase" statement puts that idea aside, I think.