YA Sci-fi Fantasy about the four horsemen of the apocalypse getting a new member. Spoilers ahead.

[Monmonez]

This is a YA Sci-fi Fantasy fantasy adventure about a teen boy who becomes the new death after the previous death runs off. The MC get hunted down by the antagonist and it turns to a race to find deaths previous artifacts. I got this book from my previous schools library about 1-2 years ago. The book was English. Now for the **SPOILERS**: I think it was the main character's teacher(?) Who turns out to be the antagonist,and the previous death. He send spherical robots after the Mc and the horsemen to hunt them down. The MC and horsemen's base of operations was the MC's shed(if I remember correctly). By the end of the story, the apocalypse is brought by prematurely, so the horsemen have to stop the apocalypse. That's about all I remember in terms of plot, but I do remember pestilence making a quip about how the MC'S initials sound like 'death', but the MC then corrects him saying it sounds more like 'deaf'.

[Mai]

This summary mentions a shed:
"In a darkly funny, action-packed adventure, fourteen year old Drake is surprised to discover the Horsemen of the Apocalypse hanging out in his garden shed. He’s even more surprised when they ask him to join them. The team is missing a Horseman, having gone through several Deaths, and they think Drake is the boy for the job."
The 13th Horseman by Barry Hutchison.

EDIT: This sentence from the book matches your description so hopefully this is it!
“What? D. F? What's that? That doesn't sound like Death!” Drake protested. “It's deaf, if anything!"