What kind of dosage are you taking? If you can run 2 miles in 30 minutes without a heart attack, the chest pain is PROBABLY unrelated to your heart. Unfortunately with a heart rate that peaks at 75, you might as well crochet for 1/2 hour for all the aerobic benefit you're getting.
If the pattern is that the pain/discomfort comes on at the highest heart-rate and abates quickly as it slows...then there's an angina scenario.
Are there any other times the chest pain occurs? Can you desribe it? Is it worse with the atenolol or better (angina pain will FAR more likely without rather than WITH the beta-blocker.)
A good test might be to pump the treadmill up to 5.0 mph and see what the discomfort picture is; try it and post results. Take 2 aspirin a day for the time being...the BIG ones. With any chest pain, better safe than sorry, and if it's a ligament pull in the ribcage, the aspirin will help THAT too.
No, a low heart-rate won't cause you pain...well not in the HEART anyway. Your LEGS might get starved for nutrients during extreme training and cramp easily but then 4 mph isn't extreme.