There is no way a pressure washer pump will drive a 4" membrane, so you can forget about those. In addition to the membrane, pressure vessel, and pressure washer pump, you will need high pressure hoses and appropriate fittings, a good quality needle valve, a pressure gauge, a prefilter housing and filter, and an assortment of valves, tubing, and fittings. Almost all of the cost will be in the pump, membrane, pressure vessel, and needle valve.
I don't know what you think you will pay in the end for all of the above, but it likely won't be much less than just ordering one of these: https://seawaterpro.com/shop?olsPage...watermaker-kit
The type of system you are pursuing runs off AC power, so I assume you have the electricity to run this - either a generator, or a large battery bank and inverter. Also, anecdotal reports from people using pressure washer pumps for watermakers show them needing replacement pumps every year or so. The pumps are cheap - $100 - but if you plan on using this watermaker for several years, the cost does add up eventually.