What state you are in? The laws vary drastically in the different states and it does make a difference.
You absolutely should not need headshots for a 4 year old, snapshots only, and they actually prefer snapshots. Once your child has gotten some work, then headshots (at a cost) make more sense, but a reputable agent will not ask you to put out money before you even have experience for such a young child.
Back when my children did some print work in California, we found their agent via the Screen Actors' Guild website. You can find a list of reputable talent agents there:
http://youngperformers.sag.org/faqshttp://www.sag.org/content/find-agent
Depending on where you live, of course. I don't even know if you are in the States. A SAG agent will follow the laws though and only charge you a percentage of what your children make, nothing up front or before they make some money and get some jobs.
I think the first tip off if an "agent" isn't a real agent is if they ask you for money or insist you take classes or something first. The two agents we have worked with both asked for a variety of snapshots taken by me to be emailed to them. I did that, then I was asked to bring in the children to be met. I was immediately told that it is not always the most beautiful or perfect looking children who make it in the industry, it is the children with personality and who happen to have a look that fills in a blank spot in their repertoire of talent (like maybe a brunette, or a red head or a boy of a certan age range or a child with a character look, whatever the case may be, just a bit of luck), and the main thing is if the child is pretty friendly, outgoing, happy, probably not shy, etc.
I will say that the age of 4 isn't a very easy age to get children in for work. 4 year olds are tough cookies (tricky stage of development) and those who cast for commercials, movies, print work, etc., know this and well, simply don't hire many 4 year olds. That doesn't mean that you shouldn't try, but I am just telling you what our children's talent agent had told us way back when.
Personally, we stopped being involved in the industry when we moved to a state that didn't have very protective laws for children in the industry. In California, the children were very well taken care of and in our current state, not so well at all. Then again, even in California, it was quite a turn-off to meet some of the parents of the children who worked frequently... stage parents like that scared the heck out of me and I knew I didn't want to risk spending too much time in such an environment and starting to think that that was normal behavior.
