1)

I ordered the ratatouille which(that) this restaurant is famous for

I ordered the ratatouille for which this restaurant is famous

2)

This is papyrus which(that) ancient Egyptians used to write on

This is papyrus on which ancient Egyptians used to write

3)

The boy who(whom/that) Jenny is playing with is my cousin

The boy with whom Jenny is playing is my cousin

4)

This is the sofa which(that) Mr. Smith talked about last night

This is the sofa about which Mr. Smith talked last night