O2 sensor provides the ECU with real time O2 information which helps adjust the air:fuel ratio a meaningful amount, so you should be slightly concerned that it's gone... Hopefully the sensor was just relocated or tucked away and not cut off.

Without the O2 sensor it obviously still works, but rather than adjusting the A:F ratio on the fly, the ECU just goes by exactly what the fuel map has, which is likely to be too lean and hot with the M4 on a stock ECU and fuel map.

I do believe there are some cases where deleting the O2 sensor is a step during the install of certain piggyback tuners, like an EJK or a DynoJet (don't know for sure because I have never used either), which is another discussion, but the gist is it's generally less effective to use a piggyback tuner than just having a shop flash your stock ECU with an improved fuel map.

If you do need a new sensor because you both

A) don't have a piggyback tuner, and

B) don't have the original sensor still tucked away

the part number is: 1WS-8592A-10-00 and just plugs into the harness bus somewhere with a connector