[Saintor]

"2C7B Oxygen sensor after catalytic converter, bank 1: Signal "

Would it be #4?

Is it easy to change or is a special tool required?

[Spectre]

It will be #4 according to that diagram. The rear or "post cat" O2 sensors are very easy to change. No special tools required. You will need a 22mm wrench to break the sensor loose then you can undo the rest by hand. Special O2 sensor sockets are available but not necessary for the rear O2 sensors on our cars.

Steps: (how it was for my 335i, But should be similar for you)
1. Jack up car safely or use ramps.
2. Take off engine underpanel using 8mm driver.
3. Bank 1 rear O2 sensor will be in plain sight.
4. Unplug the connector which is right next to it. THEN use 22mm wrench to break sensor free.
5. Hand tighten new O2 sensor, then tighten with wrench, then plug it in.
6. Reinstall underpanel cover.

Done.

Be sure to not work on the car when engine/exhaust is hot. Let it cool down for about an hour.
Also, O2 sensors are very sensitive. be careful not to drop it or bang it up during installation.

Good luck and let me know if you have any additional questions.

source: I replaced my downpipe wideband/O2 sensors last week.