Is There any way to run without them in a 1990 c4 ?

Sure you can but it's a pretty stiff fine if you get blusted. :noidea: Why not go with a high flow cat it will give you the same HP as not having one.:friday:

You can buy a new front y-pipe without cats and buy a test pipe off of ebay for $50, the test pipe replaces the main cat. Then you can lift the hood and remove the air pump and gain about 20hp after removing the 3 cats and the air pump, maybe even more HP.:burnout:

Thankyou for the advise, but what effect does this have on the computer

It will increase performance, the computer will pick up less back pressure and it will deliver more air and fuel to the engine and the engine will run more efficient and will get better milage as long as you keep your foot out of it.