I'll preface this by saying this is purely my opinion based on many years of training and experience as a trainer.
I, personally, would not perform “one more set with heavy weight stretching my pectorals as long as I can “ after 4 sets of regular flyes. Assuming hard work with heavy weights, your shoulders and pectoral muscles will be pretty exhausted. Adding heavy weight and holding the stretch as long as you can, will force your shoulders and pectorals to counteract the force of gravity. This could leave you susceptible to strains and tears of the pectoral, and, rotator cuff injuries.
Causes of a pectoral strain
A pectoral strain often occurs suddenly due to a high force going
through the muscle and tendon beyond what it can withstand. This
typically occurs during weight training especially when performing a
bench press, chest press or pectoral flys (especially with free
Sometimes a pectoral strain may develop over time due to repetitive or
prolonged activities placing strain on the pectoral muscles. This may
cause gradual degeneration and weakening of the pectorals predisposing
them to further injury. Pectoral strains tend to occur more commonly
in the older athlete and particularly following an inadequate warm-up.
And, since there's no definitive study to suggest that stretching the pectorals like you do adds any benefit, why risk injury? Especially since while lying flat, your shoulders and pectorals will act like levers resisting the weight and force of gravity.