Getting them started at that young age is great. my 11 year old enjoys it and completed a 25k ride.   I know my bike shop has several kids bikes that are by Kona, and I think Scott.  With them just starting out prehaps somewhere like Wal-Mart or Target.  Some would disagree because of the quality bike. Some questions to ask your self:  Do I want something that my child will out grow or something that will grow with  him.. Check with a local bike shop.

We replaced my son's BMX style bike from Wal Mart when he out grew it with a mountain bike from there. He is 11 I didn't know at the time he was going to get bit by cycling bug.

It depends on what you are willing to pay and what type of quality you want.  I don't think it matters at that age when they are learning and are having fun.

I would recommend going to your local bike shop. 

1.  Most have some form of trade in policy

2.  The bikes are assembled by someone with experience

3.  They can help fit your child to the right sized bike

4.  The bikes may be quite a bit lighter than the big chain store (pick up a bike at the local Costco, and you will be amazed that it weighs a lot more than yours)

5.  You are forging a relationship between your child and bike shops!

Trek makes a bike that the cranks/pedals come off.  This is ideal for teaching a youngster to  ride without training wheels.  Part of the scenario right?  The saddle is lowered so that the child can sit on the seat and "walk" the bike and then glide and coast with it.  Training wheels inhibit learning to balance the bike and steering as well.  Most of the kids already know how to pedal so what they need to learn is to balance and steer.  Make sure if you use a preexisting bike that it has a hand brake or the kid is totally  away from traffic or obstacles or hills.  They still need to know how to stop.  If you have an older bike, take off the pedals at least and lower the saddle. This should work but again make sure they are in a safe area. 

EJ LEVY

HI,

I presume by now that you have purchased a bike for your 4 year old. I presume that you also purchased a helmet. You might also consider gloves and glasses for even a small child. Impact and UV protection glasses for safety can't start too young.  While in a previous message I noted the Trek bike that has removable crank arms, you mght also consider getting a bike with grow cranks that have inner and outer holes for the pedals.  I think it is nice that BOB's kid just jumped on a bike and pedaled away but  that is rare.  Also, a 4 year old is not a 5 year old and getting a bike that is too big with the anticipation of having it more that 2 or 3 seasons is not really reasonable. While I'm not suggesting that is what BOB is saying, it could be construed from his note.  Sell the used bike when done or donate it to some local program for underpriviledged kids. 

Have fun.

EJL

Agree too on bike size.  The bike that fit my 4 year old last year may not make it pass this summerwe'll see based on how he grows.  Also, every kid is different.  My youngest learned at 4, but my oldest wouldn't have anything to do with learning until 6+.  One more thoughtI think learning to use a scooter first helped with the bike learning process.

We got our son (2) the Specialized Hotwalk becasue we were told this type of bike would help him develop balance and comfort on a bike. After 6 months he can get on and off of it, ride at a decent speed and even ride it backwards (although I tell him to stop becasue he's just showing off. 

http://www.specialized.com/bc/SBCBkModel.jsp?spid=38486&eid=164