Microsoft Universal Folding Keyboard
If the Surface Type Cover is too much of a hassle to carry, buy the Microsoft Universal Folding Keyboard. It's just 0.19-inches thin and doesn't take up much space in your bag.
As its name suggests, when you're ready to work on your Surface Pro 4, unfold this keyboard and lay it flat on the table. Its keys mimic the square plastic keys of the Surface Pro 4 Type Cover. It connects to your Surface Pro, your smartphone and your tablet through Bluetooth at the same time. There's no need to pair it again everytime you switch devices thanks to that feature.
You don't need to spend hours hunting for very small batteries at different drug stores either. The Microsoft Universal Folding Keyboard recharges through an included MicroUSB cable. A single charge should last for three months of typing.
Microsoft sells the Universal Folding Keyboard for $99.99.