Engine swap
 

I have a 2003 toyota camry 6 cyl and was wondering if I could put my motor in a 2004 toyota camry 4cyl

It may be a lot of work.

The usual way to do this is would include swaping the 2004 4-cylinder computer for the 2003 V-6. However the 2004 V-6 was upgraded to have a different generator, A/F sensor design, fuel cut off system incorporated into the airbag system and made to comply with more restrictive emissions requirements. The cars brake system was changed to CAN and the engine immobilizer design significantly changed. The 2003 computer will not be current to these changes if and how they interact with that computer.

If you use a 2004 V-6 computer you will need to find one that comparable with that specific of you car and upgrade the engine to 2004 standards, You will still need to change the wiring harness, etc. .