Yes. However, you must use the clutch release mechanism from the 5 speed and you will need to fashion a jumper wire to ground for the neutral switch. The 4 speed neutral switch grounds inside the transmission. You will also need to use the shift lever from the 5 speed. Many years ago I installed a 5 speed from a 1974 R90/6 in a 1973 R75/5. It was a worthwhile upgrade for that bike.

I forgot to mention that the final drive has a ratio of 3.09. I noticed that the five-speed transmission has a different first gear ratio (steeper) than the standard four-speed. Do you think it will be harder to go from a stop (clutch control wise) with the five-speed? Any disadvantages in using the five-speed? Thanks.

My un-scientific instinct tell me that you might not even notice the difference. You might compare the difference of the 1st gear ratios and calculate that difference as a percentage to quantify the change you are contemplating. I never gave it a thought when I did the install on my /5.