Signet Ring Cell is very rare. Yes, it's bad but again, very rare. Believe me, they tested your 4 cm polyp and it had to be fine or you would have heard back very quickly. Less than 1% of all CRC's in the world are SRC.
If 149,000 people are dx'd each year with CRC, 1% of that is less than 1,500 out of 360 MILLION people in the U.S. have SRC.