Venturi vac @ the TB is completely unrelated to the vac in the manifold. They will vary wildly.

A drop in manifold vac like that either indicates a leak or another issue (like excessive backpressure as suggested, valve timing off or valve clearances off, etc). Ignition timing still spot-on? Sudden failure of the aux air valve? If it were a big vac LEAK I'd expect a raise in engine idle speed. Is it faster? 4" is enough to notice faster idle unless it's caused by something else...