question for the swim gurus

uh, yeah, breathing is kinda important. Otherwise, you don't get any oxygen and you die.  And that ruins your whole workout.

Every 4th stroke?  Hmm. I have always heard 3  or 5 is optimal. By breathing on both sides, your roll is more balanced, and for triathletes, the ability to sight on both sides is also helpful. 

Breathing is key, but it changes depending on how far you're going.

If I was racing a 50 sprint I'd breathe 2 times down 3 times back. 

 For just about every distance over 100 yards I take a breath every 3 strokes (which of course alternates sides).   Somtimes I'll do a breathing drill where I'll swim, say a 200, and breathe on the 3rd, the 5th, then the 7th stroke, then repeat for the whole set.

Bottom line is you should be comfortable when you swim. Nothing will tire you more quickly than to struggle. If you're at your best breathing every stroke on the right side or the left side or out a blow hole in the top of your yead, then do that and don't worry about what others say or do. Since you're paying an instructor for help, however, you should probably try to do what he/she says - it might just work for you. And it wouldn't hurt you to learn to breathe bilaterally for short durations for sighting in a race, but do what works best for you.

My .02 only.  The MOST important thingf for swimming IN TRIATHLONS is not to go into oxygen deficit on the swim (different if it's just a swim).  You don't want to start building lactic acid on the swim, so that by the time you get on the bike you are close to spent. Thus, if you need to breathe every second stroke, breathe every second stroke.  If you can do every third comfortably, do that.  Whatever it is YOU can do, do it. 

 If you starve your body of O2 in the swim, you will go anaerobic during the swim and might screw up the rest of your day.

Usually instructors/coaches do this with a purpose. Sometimes by breathing every 3^rd or 5^th stroke it forces you to work a bit more on your reach allowing you to keep you leading hand in front and gliding a bit more. (As you are not rushing to pull and roll to breathe). Also it might help you to find a better rhythm for your rolling from side to side and/or your kick. Finally, it might be because he wants you to become more efficient when breathing by making you swim holding your breath a bit longer than usual. Having bilateral breathing is a great skill to have and master for OWS regardless of your goals. In my case when I am racing, I usually breathe every other stroke and after 10-12 strokes I switch sides, still during my swim workouts I work my breathing every 3^rd or 5^th stroke during warm up/cool down.

trust your instructor…

okay....  training and racing are different.  Obviously you want to train how you race, but not all training sessions are race sims.

I breathe according to pace.  For a fast TT I by be forgetting the bilateral and going every 2 strokes, just to make sure I'm getting enough air.  I may go all right on one lap and all left on the other, but I just do what lets me have the most air and be comfortable.  Sometimes I'll even do 3 or 4 for a ways, then a few at every 2.  Whatever I feel I need to do.

When I'm doing stroke focused drills, especially catch drills, I will push it out to every 4 or 5 strokes.  Breathing takes a lot of mental concentration and tends to take focus off of form.  I find it easier to perfect my stroke if I breathe less.  Of course I am going slower, so my HR is lower and so is my demand for oxygen.  You will also find by doing this that your actual demand for oxygen is much lower then you think it is.  It's just like running.  When you are going full out at LT pace, you tend to breathe every 2 foot falls and you pant yourself to death.  If you focus and force yourself to breathe every 4 strides then you become more relaxed and breathe easier at the same pace.

and "not breathing enough" won't build "lactic acid" in you system.