I am curious to know why 4 unit new porcelain bridge on molar teeth breaks that easy while just eating soft sandwich. what should i replace it with.

Bad bridge.: A new bridge should not break eating a sandwich. Something was wrong w/ its fabrication. If you don't want another bridge, the very best way to replace missing teeth, if possible, are implants supporting single crowns. They are easier to clean, never get cavities, look great and feel natural. That's the way i'd replace any missing tooth!

Implants VS Bridge: It was not mentioned how old is the bridge. Porcelain bridge is a second choice to implants. The chewing capacity is more superior with implants like your original teeth grew back. Oral hygiene won't be compromised because it is easier to floss!

Porcelain on teeth: Any time you place porcelain on crowns especially the molars, you are running a risk of fracture. It happens in 10-20% of cases. The fractures start due to a variety of reasons but the break can happen with the most unlikely fuse. There may be metal under the bridge protecting the tooth. Replacement may be a bridge with metal oclussion, new zirconia material, or implants to replace teeth.