The present invention is directed to compounds which are beta-lactamase inhibitors. The compounds and their pharmaceutically acceptable salts, are useful in combination with beta-lactam antibiotics, or alone, for the treatment of bacterial infections, including infections caused by drug resistant organisms, including multi-drug resistant organisms. The present invention includes compounds according to formula (Ia): or a pharmaceutically acceptable salt thereof, wherein the values of R1, R 2, R 3 and R 4 are described herein.
