An iP4000 is without question worth saving.
It is the last of the chipless tank generation of Canon printers and is highly prized working or not (parts).
If the carts were left out of the head for any period of time the head could have dried up. This is usually recoverable.
Also the purge pump could be clogged, again repairable.
With all colors failing to print, I would look at purge pump function.
Without any flow of cooling ink, be careful not to overheat the head with cleaning cycles.