Most 160A-200A rods will burn a 4MM rod, the question is how long do you want to run 4MM rods for?It'd be hard get a good clean oil cooled welder. We looked at a few and they weren't too clean . What would be a good inverter to buy to run a 4mill rod
Worth checking underneath for corrosion on case. look for oil seepagejust because an oilcooled welder case looks grubby and the leads are rotten don't dismiss it if the price is right . the inside will probably be like new .mine looks tatty and done a fair bit but will run another 45 years renew the leads clean and paint and it will be like new they can be had from 70 to 120 quid 180 amp