I have a much larger bot that is called the RoboServant that uses 4 continuous rotation servos to drive it.
Here is what I did to make this work.
The two servos on the right side and connected together using a Y-cable as well as on the left side.
RoboServant is much heaver than what you are currently doing so the servos should be fine.
The servos I used on RoboServant were inexpensive plastic gear ones that I picked up from Radio Shack but from my research are produced by parallax.
He stands 43 inches (109 cm) tall and with the tablet on him weight right around 7 lbs. I don't think you will have any issues with what you are doing.
One other thing I did to help in the movement was to add two Omni Wheels to the rear, this compensates for there not being an actual pivot wheel in the back like these http://www.andymark.com/PhotoDetails.asp?ShowDESC=N&ProductCode=am-3047
You can use four standard wheels but you will see that when turning a lot of force is then applied to the rear wheels which over time can cause the servos to fail.
After looking closely at your images I noticed that you have the servos facing each other. You will need to make the servos so that they are going the same direction otherwise they will fight each other and you will sit in one spot and spin.