Excel: Multiplier in a IF/OR statement

First time posting, so apologies if this is in the wrong place or is some variation on a question asked before. I had a quick look round and couldn't find a similar question.

I'm doing a workload model for academic staff in my school. I need to multiply an existing column (i) that has how many hours each teaching session lasts by either 3 hours if column c="Seminar" or 5 hours if column c="Lecture". What I've got, copied below, comes back with "You've entered too many arguments for this function".

=IF(OR(C4="Seminar"),I4*3,I4, IF(OR(C4="Lecture"),I4*5,I4))

Thanks in advance for any help.

Paul