My electrician is installing a tankless water heater he told me to get 4/2 AWG with ground for 240v. I went to Home Depot and they have no idea what I’m talking about. Can someone tell me what I need to say to explain what I need?
This is better done with conduit
As it turns out, 4/2 NM-B cable isn't even made; you could use a 4/3 NM-B cable, but that's upwards of $4/ft. You are much better off wiring this using a 1" ENT ("smurf tube") with a couple of 4AWG THHNs for the hots and an 8AWG THHN ground. Even in copper, this costs less than $3/ft; if you went with 2AWG aluminum instead for the hots, you could get the costs down to under $2/ft. Just remember that with conduit, you'll need to insert boxes or bodies as-needed so that you don't have more than 360° of bending between pull points.
An extra bonus is that if you decide you want something else there instead, it's now trivial to pull a neutral over and drop a subpanel in. Beats having to rip out and replace an inappropriate cable, eh?