specifically a horizontal 2x4, used to attach vertical 2x4 framed wall at
the floor

Sounds like the bottom plate of a 2X4 wall.

Put 4" foam sill gasket down under the plate and use 2" concrete nails.
Drive them thru the "2 by" until they re flush with the bottom. Line up
your wall and pound the concrete nails "home" with a sledge hammer. One
good BANG ! each should do it