My son went to the local Kumon centre once a week for two and a half years from age 8 to 11. Based on his experience, I would thoroughly recommend it. We chose it because he was struggling in maths through lack of confidence - not lack of ability. For whatever reason, he was struggling at school and needed a bit of extra help of one sort or another. We chose Kumon as the most attractive of the possibilities we looked into.
Kumon uses simple repetetive exercises to help the student get used to the "feel" of numbers. One of my son's biggest difficulties was learning his multiplication tables. He is now supremely confident in maths generally, and has very fast mental maths. He also got to the final of a regional schools maths competition. Not something he was encouraged to do by pushy parents, in case you're wondering - we didn't even know about it until the invitation came through the door.
However, to qualify my endorsement, I would be wary of recommending it for a 4yo because I don't know how a child can benefit from it at pre-school age.
By the way, Clarion - Kumon didn't replace parental support in my son's case, it supplemented it and helped him in ways that I am not qualified to. The way Kumon is structured, it just doesn't work without proper parental support. And I never once felt "locked into the system", nor pressured by the people at the local Kumon centre. In the end, he left because we couldn't fit it in with all his other activities, and we felt that by then he'd got the benefit he needed from it anyway. But there was no difficulty leaving - we just gave notice and cancelled the direct debit.
Sure there are other methods that are cheap or free, but based on my experience, I would say Kumon is good value for money if your child needs extra maths help.
d.