Planning trip to Paris in October. A friend recommended staying in the 4th arrond. but another rec'd the 5th.

What is the difference between the two?

Generalities are always difficult, but overall, the 5th is younger, livelier, more budget oriented. The 4th is a little more arty/slightly more upscale. It has a higher concentration of gays and Jewish population, and also includes the "well heeled" Isle St. Louis.
Either can be very good. I'd base the decision more on the actual hotel or apartment than the general area.

The 5th contains the Sorbonne. I love students, I love the area, I would skip it until I'm persuaded they aren't going to be demonstrating in the streets again next year.

It wouldn't be a trip to Europe if someone wasn't demonstrating in the streets! We quite look forward to that....

I just don't like it under my hotel window.

If you want to live among fellow tourists (especially American ones), go to the 5th, they love it. If you are crazy about noisy pedestrian streets with sun burnt Swedish girls getting drunk on retsina in cheap Greek restaurants, head for the 5th.

I think Neo has summed it up pretty well.

For our upcoming trip to Paris I posted a message about choosing a hotel and did a lot of musing over which hotel just within ONE area because one area can have many degrees of variation.

Last year we stayed in a wonderful hotel but I thought the very immediate area around it was dreary but I suppose those sorts of "problems" only come when you've been to a city several times.

What kind of atmosphere are you looking for..? (quiet at night, close to clubs/bars, elegant shops, wide boulevards, narrow streets...?)