Well if he was selling it for less than $4000 then it probably was just a standard milspec AR lower receiver - and he was using an improper term. Now if it was a complete lower then M4 could mean it has a standard collapsing stock like an M4.
Lazy term on his part anyway, so don't be ashamed to have him describe exactly what it is. Call, if it's 7075-T6 alloy and the price is good you might want to get over there quick if you need one.
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