What is Ar4s1?

Answer 1

It should be read as: [Ar] 4s1 .

Meaning: the same electron configuration as Argon(18), expanded with ONE (1) electron in the 4s orbital.

So it is one electron, thus also one proton more than Ar(18): this makes it atom number (19), which is named as Potassium:

K(19): electron configuration [1s2, 2s2 2p6, 3s2 3p6] 4s1 ,

(between squared brackets [...] is the electron configuration of Argon)

Answer 2

Ar4s1 refers to the electron configuration of an argon (Ar) atom in its ground state. It indicates that there are 4 electrons in the 4th energy level and 1 electron in the 5th energy level of the argon atom.