“My 4 year old son recently has an irregular heartbeat. Would he need a transplant in the future?”

Not necessarily

The need for a heart transplant is evaluated on many factors, most important of which is heart function. A irregular heart beat may be a manifestation of a weak functioning heart. A pediatric cardiologist who specializes in this field will need to moniter his heart function, most likely with periodic echocardiograms, and assess his response to medical treatment.
If he ultimately needs one, his doctor will place him on a transplant list which has many factors to determine a possible match for him. Only after he has surgery, will they determine if accepts the transplant.

Irregular heartbeat, especially in children, can be caused by many things, only one of which is congenital heart defect and only rarely would that require transplantation. I would talk to your doctor(s) about what the specific diagnosis of the irregular heartbeat is, before jumping directly to questions about transplantation. If indeed the irregular heartbeat is caused by congenital heart disease, frequently that can be repaired and your child would be expected to have a completely normal life.
Transplant immunology - what you asked about regarding rejection - is a very complicated subject - and for solid organs - liver, kidney, pancreas, heart, lung - is a life-long risk, and although it diminishes over time, requires long-term anti-rejection medications.

You are jumping way ahead. Your son needs an evaluation by a pediatric cardiologist who will work up his rhythm problem and seek advice if needed by an expert electrophysiologist. Only at that point will they know whether any treatment is needed.

That is a hard question to answer. It depends on what the underlying cause is. I would have to know that to give a reasonable reply.
Most transplants are well tolerated with long-term survival, but require very strict medication reliability. Rejection of a new heart is rarely fatal and can be treated by medicines. Matching a new heart well up front is critical.

Good luck,

Dr. Stam