John L has correctly stated that some 4x4's are exempt from the requirement regarding towball heights. To be exempt, the vehicle must be classified as off-road type.
According to EU Directive 2007/46/EC, an off-road vehicle is defined as follows:
4.1.
Vehicles in category N1 with a maximum mass not exceeding two tonnes and vehicles in category M1 are considered
to be off-road vehicles if they have:
- at least one front axle and at least one rear axle designed to be driven simultaneously including vehicles where
the drive to one axle can be disengaged,
- at least one differential locking mechanism or at least one mechanism having a similar effect and if they can
climb a 30 % gradient calculated for a solo vehicle.
In addition, they must satisfy at least five of the following six requirements:
- the approach angle must be at least 25 degrees,
- the departure angle must be at least 20 degrees,
- the ramp angle must be at least 20 degrees,
- the ground clearance under the front axle must be at least 180 mm,
- the ground clearance under the rear axle must be at least 180 mm,
- the ground clearance between the axles must be at least 200 mm.
4.2.
Vehicles in category N1 with a maximum mass exceeding two tonnes or in category N2, M2 or M3 with a maximum
mass not exceeding 12 tonnes are considered to be off-road vehicles either if all their wheels are designed to
be driven simultaneously, including vehicles where the drive to one axle can be disengaged, or if the following
three requirements are satisfied:
- at least one front and at least one rear axle are designed to be driven simultaneously, including vehicles where
the drive to one axle can be disengaged,
- there is at least one differential locking mechanism or at least one mechanism having a similar effect,
- they can climb a 25 % gradient calculated for a solo vehicle.