On a car as light as a 4/4 , anywhere between 18 and 22 psi is about right for road use. If used on the track then go to a much higher number to prevent the tyre rolling off the rim, say 28 to 30 min.
As others have said heavy duty tubes are a first port of call , but even this may not help if the insides of the tyres have very abrasive surfaces.
Are you using rim bands inside the wheel to protect the spoke heads (I assume you have wires?) This can also help.
Long term , go for new wheels and fit tubeless tyres as per Tim (Hamwich)
