Like… This is Numbuh 4. A full-fledged operative of the KND, a group of specially-trained children who are cunning, capable, battle-smart and highly adaptable. He passed through Base Camp, through the Final Exam (which is far from easy, mind you- the Numbuh 1, head of Sectuh V and highly-regarded operative, didn’t pass his Final Exam) got his Numbuh, and has been a steady, reliable source for the team.
And he just… kicks this thing open??
I know Wally doesn’t always have the best plans, especially early on in the show, but he cares a lot for his team, and would never intentionally put them in harm’s way. So did he just assume they’d all be able to push the huge, heavy pianos out the back of the truck?? I know he didn’t know about the hill, but what if they turned?? What then??