Introduction:
Isomers are compounds that have the same molecular formula but different arrangements of atoms. In C6H12 with a 4-membered ring, there are multiple possible isomers.

Types of Isomers:
There are two types of isomers that can be formed in C6H12 with a 4-membered ring:
1. Constitutional isomers: These isomers have the same molecular formula but a different arrangement of atoms.
2. Stereoisomers: These isomers have the same molecular formula and the same arrangement of atoms but differ in the spatial orientation of their atoms.

Constitutional Isomers:
The number of constitutional isomers that can be formed in C6H12 with a 4-membered ring can be calculated using the formula 2^n, where n is the number of possible ways in which the atoms can be arranged.
In this case, there are 3 possible ways in which the atoms can be arranged:
1. Four carbons in a ring with two methyl groups attached to the same carbon atom.
2. Four carbons in a ring with one methyl group attached to each of two adjacent carbon atoms.
3. Four carbons in a ring with one methyl group attached to one carbon atom and a double bond between two other carbon atoms.

Therefore, the number of constitutional isomers that can be formed in C6H12 with a 4-membered ring is 2^3 = 8.

Stereoisomers:
Stereoisomers can be further classified into two types:
1. Geometric isomers: These isomers have the same molecular formula and the same sequence of bonded atoms but differ in the spatial arrangement of their atoms due to the presence of a double bond or a ring. In C6H12 with a 4-membered ring, there are no geometric isomers.
2. Optical isomers: These isomers have the same molecular formula and the same sequence of bonded atoms but differ in the way they rotate plane-polarized light. In C6H12 with a 4-membered ring, there are no chiral centers and therefore no optical isomers.

Conclusion:
In summary, there are 8 constitutional isomers and no stereoisomers in C6H12 with a 4-membered ring.