No. 4 sensor is the far one on the drivers side (on a UK car).
It is possible to change the sensor without removing the bumper if you drive the sensor into the bumper with a drift which will enable you to remove the collar and then the old sensors with long nose pliers, you can then fit the new sensor to the collar and re-install.
That's not the prescribed method and there is a small risk of damage but I've done it successfully 10+ times and it saves hours....
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2011 XC90 D5 Executive
2003 C70 T5 GT
2012 Ford Ranger XL SC
1977 Triumph Spitfire 1500
1976 Massey Ferguson 135
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