Hey there,
I'd defo go with the 4 and not the three. The Pro 4 has an Intel Core processor, versus the Intel Atom processor in the 3. The Core m3/I5/I7 are much better. If you can get the I7, it's a pretty strong CPU still. The 4 comes with a bigger SSD and ram options too.
The Surface doesn't have an ordinary bios per se. It's rolled out by MS, and so you can't manually change anything. in it. It gets updated in very much the same way as a Windows update. It's fool proof.
Yes, you can use the Surface as a laptop. That's it's primary function. But it's a 2 in 1, so performs well as a tablet too. I've the Pro 6, and I love it for work.
Although the Surface Pro's don't have great GPU's, you can stream on the ones with good CPU's. I stream from my Steam account on my desktop to the Pro 6, and it works like a charm. So you can have a little fun, whilst being productive.
The screens on the Surface are really good. They are high res 4:3 with ratio. You can fit two apps split screen and it works well.
They keyboards are really good. Make sure when you buy the Surface it comes with the keyboard, and pen if required. Otherwise they can cost a bomb to buy new.