There are some areas of QubesOS that are a little bit obscure, and the storage pools (particularly if you have more than one, which is quite normal if you have more than one disk) are one of those areas.
List your existing pools with:
and find out the name of the actual thin pools that Qubes 4 uses with
which, if you are curious, can then be correlated (one each) with a particular LVM logical volume listed thusly:
At this point in time, the Qubes 4 installer is creating a default thin_pool named "pool00" which contains dom0 and any appvms that were created in the normal way without explicitly specifying a different pool.
To specify the pool which holds a new appvm, use the -P switch:
qvm-create -P platter_qubes --label blue
where "platter_qubes" is my secondary / storage big spinning disk, not my little SSD holding dom0.