I have an electric hammer. Plug it’s cord into the electric faucet, lift it up, and it drops down on the nail by itself! Great invention.
All kidding aside, it depends on what you want to use it for. A 4 volt battery is not going to do much for you. It will be hard pressed even to do simple jobs around the house. I would recommend at least a 12 volt battery, and really, if you are going to be using it much outside, especially for building track roadbed, I would get an 18 volt battery.
Unless you are a pro, using the thing every day, don’t waste your money on lithium. NiCad is fine. Home Depot has a great Ryobi 18 volt drill/screwdriver, two batteries and a 5 1/2 inch circular saw for 49.90. At that price, you are practically stealing the thing. I have the same kit and love it.