Infinitely many.
4
There are only four different configurations.
Ignoring rotations, there are 3 distinct solutions.
A rectangular prism that is 4 cubes by 2 cubes is made up of 8 cubes.
24 of them
13
There are only four different configurations.
If you have got enough cubes, as many as you like.
Ignoring rotations, there are 3 distinct solutions.
You could try 14!
3
A rectangular prism that is 4 cubes by 2 cubes is made up of 8 cubes.
24 of them
about 10
25
With no repetition of shapes (symmetry) we have obtained 26 different combinations.
Well, honey, if the height is 4 cubes, that leaves you with 12 cubes to work with for the base. You can arrange those 12 cubes in various ways to form different rectangular prisms. So, technically speaking, there are multiple rectangular prisms you can create with 48 cubes and a height of 4 cubes.