How can I regroup existing several LiDAR classes into the only 4 classes the client needs?

Morning,

I have a group of LiDAR point clouds which were classified in several classes.

Unfortunately the client only wants the point clouds classified into 4 groups.

How can I regroup several classes into the 4 classes I need? Example (class 1-2-3-4-5 to become class 1, class 6-7-8 to become class 2, 9-10-11 to become 3 etc etc)