My 4 yrs old kid was just diagnosed with a heart murmur. Is it something serious? Or is it common on kids?

Probably normal. Most murmur "bad" will be picked up on day one or rarely within the first 4-6 mo. Over the years I found that if I listen to any normal kid long enough, I will find an extra sound in most. Most are evidence of blood flow, but some are just the dynamic action of the heart within the heart sac. If doc is curious s/he may get an ultrasound to rule out a problem.In most, this is not necessary.
Heart murmur. A murmur is an extraneous heart sound. It may be functional in nature. An organic murmur would require attention of a pediatric cardiologist. Further evaluation is needed.
Likely normal. Having a heart murmur means that when the heart beats, there is enough energy in the blood flow that some of the energy is transformed into sound. In a six year old, most likely this is a normal murmur and not a murmur caused by a heart problem. An experienced listener can hear differences in the sound that predict whether the murmur is normal. Children typically have normal murmurs.