According to my 4 yr old, I would go with Bumblebee, he is the yellow transformer, as a car he is a Camero. How elaborate do you want to go? Do you want to do a 3D? For my 4 yr old, I would do some cake transfers of Optimus Prime and Bumblebee. (but that is according to my 4 yr. old.) :0)
I have one to do this weekend, have done a few in past. They are the ones I dread the most!! But I print out a coloring page and do a fbct. I also plan on doing the yellow one.
Here's what I did for my 5 year old last year. I wasn't happy with the cake itself but I thought I did pretty good on the cookie (Optimus Prime) and the chocolate transfer (the autobot symbol).
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1171031
You don't say if this is for your child or sale though. If it's for sale I'd play it safe and get some toys, or have the parents purchase them, to place on the cake or look into purchasing an edible image. I have seen rings meant for cupcakes with autobot and decepticon symbols and an Optimus Prime toy at a cake store. The toys on my son's cake can be purchased at most stores that sell toys, they're called Robot Heros and come in two packs.
Find out exactly what transformers the child is into right now too if you don't already know. There is the live action movie but they also have a series on Cartoon Network called Transformers Animated. My youngest likes both but went nuts when his grandma found a transformers animated t-shirt.
julitre1 is right about the characters, Bumblebee and Prime are the two really big ones right now. Bulkhead is the other that my son really likes right now, he's an olive green thing on the animated show right now and must be big since he's with Prime and Bumblebee on my son's t-shirt.
With two boys I know way more than I want to about all of them. 
Here are a couple that I've done,,,,bumblebee and optimus prime.
http://www.cakecentral.com/cake-photo_1191650.html
http://www.cakecentral.com/cake-photo_1214626.html
I did these using a coloring book page,,and I still have them. If you're interested just email me.
Thanks everyone for your ideas...I'm still quite the beginner at this but would like to know what is FBCT? This cake is not to sell it's for my godson. He loves Bumblebee and Optimus Prime which seems to be the most popular ones...I would love to do an image on my cake but unsure how to do that....
For the future, you might also want to find out if they are wanting the new animated version, the film version or the 80's animated version. I know my Husband love the 80's version, and would not let me down if I made a cake with the new animated version, although the film version seems to be safe ground no matter what.
As for this cake, I would get a coloring book and find a good picture with Bumblebee and Optims Prime. Get a reverse copy of it and either do a FBCT or a chocolate transfer. They are pretty easy to do and there is a tutorial on how to do the FBCT. HTH
This maybe look kind of difficult but you can also see the picture and the cutting instructions by the decorator. I love it...
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1251771
She also made cupcakes of the theme
If they are his favs, they are the good guys, the Autobots. You could do a FBCT of the Autobot logo
http://www.hasbro.com/transformers/default.cfm?page=News/Item&newsid=EA145317-D56F-E112-446782EEFE59E513
the red one is the Autobots.
HTH
I just did one, I didn't see the movie but I think the character is called "Bumblebee."
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1263223
Here is one that I did for my nephew. I know that I got some of the ideas from cakes on this site. Little boy cakes are so difficult because they usually involve a lot of detail. My nephew was just so happy to get 2 toys on it. He loved it! Good luck!
http://imgsrc.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1264526&done=1
maybe this link will work
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