Learn Dutch: We zijn met z'n vieren in de escaperoom. - The four of us are in the escape room.

In modern Dutch, met z'n... has become a fixed expression to describe the size of a group, regardless of who is in it. Even though z'n originally comes from zijn (his), you use it for we, jullie (you plural), and zij (they). You might occasionally hear met ons vieren, but met z'n vieren is much more common and completely correct.

Why is there an -en added to the number vier?

In this specific grammar structure, the number gets a plural ending, usually -en. It turns the number into a noun that represents a group of that size. So vier becomes vieren. This happens with almost all numbers in this structure, like tweeën (twos) or drieën (threes).

Could I put met z'n vieren at the end of the sentence instead?

It is much more natural to put it where it is: before the location. Dutch generally follows a Time-Manner-Place word order. Met z'n vieren describes how you are there (Manner), and in de escaperoom describes where (Place), so the group size comes first.

Why is escaperoom written as one word?