I've got an existing 3" dia bathroom fan vent going through the roof. I want to switch it to 4", is it advisable to have a reducer (4x3) to connect to the exhaust at the roof end?

Switch what, the fan or pipe? If it's the fan, just get a 4 x3 reducer and install at the fan,leave the pipe 3". Not that much static pressure to worry about.

I want to switch the fan and the pipe, but if it's not a big deal to leave the 3" pipe, then I'll just get a 4x3 reducer and install at fan. With the 3" should I worry about not pushing enough air out since now the fan is larger?

I want to switch the fan and the pipe, but if it's not a big deal to leave the 3" pipe, then I'll just get a 4x3 reducer and install at fan. With the 3" should I worry about not pushing enough air out since now the fan is larger?

Not much back pressure to worry about. Save your money and put the 4 x3 at the fan. I just did one like that and it works great.