We own a 3-liter, six-cylinder, 145 horsepower Ford Ranger which we bought to pull a small, 3500 pound (loaded) 5th-wheel travel trailer. With trailer in tow the Ranger struggles a bit in the hills, but can usually maintain 55-60 mph. When we're not towing it has way more power than we need. We have a friend that tows his nearly identical 5th wheel trailer behind a 4-cylinder 2.7 liter, 150 horsepower Toyota Tacoma; he reports very similar results.
The point is it's not the number of cylinders your car has, but how much the car and its occupants/payload weigh vs its overall horsepower. If you have a big, heavy car hauling big, heavy people and use your air conditioning while driving in the rockies, a 3-cylinder, 1 liter, 55 horsepower Geo Metro is not going to do it for you, but unless you're hauling really heavy stuff (or really heavy people), a 150 horsepower engine is more than sufficient to get you there and back again.
Speaking of my old 3-cylinder, 1 liter, 55 horsepower Geo Metro, it was generally plenty zippy, too. With four adult passengers it didn't do quite so well, and loaded to the rafters and sagging on its suspension as we moved from the San Francisco area up to Washington State it struggled to maintain 45 mph in the mountain passes, but even heavily loaded it got us where we were going.