What are the only four states that share a common boundary?

Answer 1

Texas Arizona California & New Mexico

as they share the Mexican boundary in common

however they also share it in common with half a dozen Mexican states

so it should be noted that the question & answer are defective in this regard

moreover the question probably does not intend this answer at all

but rather appears to refer to a single boundary point known as the four corners

which is shared in common by Utah Arizona Colorado & New Mexico

so it should also be noted that a boundary & a boundary point are not quite the same thing

since a boundary is generally defined & considered as a line rather than only a single point