The four O2 sensors on the exhaust manifold allow the engine computer to monitor the efficiency of the catalytic converters by analyzing the readings from the upstream sensors and downstream sensors located before and after the catalytic converters.
Moving the oxygen sensors to in front of the catalytic converter will interfere with their ability to correctly measure the emissions and can cause the engine to run poorly or even damage the catalytic converters.
Additionally, it can cause engine damage such as overheating of your catalytic converter.
If you are experiencing problems with your catalytic converters it is best to have a qualified technician diagnose the problem and perform any repairs that are necessary.