Can you be a little more explicit and detailed? You exposed HP5+ sheet on the baseboard of your enlarger? How did you determine the exposure? With a light meter? Placed on the baseboard, looking towards the enlarging lens? A light meter normally requires one to set the ISO (ok, 400, or whatever...) and an f-number... but you are measuring in the plane of the film, the light seen by the film, not the light seen by film behind lens, as in a normal situation. I don't understand what exactly you did. When you say EV4, do you mean like 1/2s @ f/2.8? But if you measure at the film location, the f-number is already accounted for as it determines the amount of light as measured.Measured EV 4 on the base board.
That is really short!Developed the 1st sheet for 4 minutes(normal 12min...)
IF you managed to properly meter in the plane of the film (see above remarks) for normal (i.e. zone V) exposure, "half" of the Stouffer wedge should be, IIRC density 1.5, i.e. 5 stops below that, just reaching Zone 0: it is quite normal that the density (above film base+fog) goes down to zero at about that point.development 11min as it is closest to normal fopr a sanity check, and about half are visible.