I have a 9 year old who has never been dry since potty training. I take his to a pediatric urolgist - but he says that 20% of kids will outgrow bedwetting each year up until age 15 - that they really don't get worried about it this young. My son has had lots of tests - bottom line, his bladder capacity is really low (like that of a 2 year old) not a 9 year old. He is on medication - has not made any difference yet - we have another follow up later this month.
There are alarms - which tend to have the best result - but the child MUST want to use it or it will not be succesful.
There are MANY reasons children wet the bed: hormonal, underdeveloped sphinter control, small bladder capacity, psychological - therefore there is not just one solution. I am glad for some that the motivation of a sleep over helped them become dry - for my child it caused horrible anxiety - b/c he has no control over being able to wake up dry!
I probably at age 4 would not be aggressive in treatment - unless he has been dry - and now is wetting again.