According to the below table when backspace is 4,75" ET will be on the level of 18 mm, with backpace 4" ET will be 0, and with 4,25" ET will be 6.https://www.rsracing.com/tech-wheel.html
Why then should I buy wheels with backspace 4,75" and use the spacer, if I can buy a wheel with lawer backspace? For my Mustang normally I should use wheels 16x7" backspace 4,25" according to the table:http://www.mustangunlimited.com/wheel-tire-fitment.html
The question is if I can use backspace 4" instead of 4,25". I would like to lower ET not to increase it as backspace should be used at higher ET.