Learn French: Ne vous approchez pas du four, il est très chaud !

In negative commands, the reflexive pronoun (vous) stays in front of the verb, safely wrapped inside the ne ... pas. You only move the pronoun to the end of the verb with a hyphen for positive commands, like Approchez-vous !

Where does the du in du four come from?

In French, the verb for "to go near" or "to approach" is s'approcher de. Since the word for oven is masculine (le four), the preposition de and the article le (the) combine to make du.

Could I say c'est très chaud instead of il est très chaud?

You should use il est here because it directly points back to a specific masculine noun (le four) and is followed by just an adjective (très chaud). You generally use c'est when followed by a noun, like c'est un four chaud (it is a hot oven), or when making a general statement not linked to a specific object.

Is approcher always a reflexive verb?