Weight Reduction 4 is for some of the more expensive road cars, and is purchasable. I don't have my spreadsheet open at the moment, but if memory serves, those cars, and only those cars, were capable of getting WR 5. In ticket form, WR5 bypasses the need to buy stages 1-4 in order.
I'm actually expecting to sell a bunch of cars when I get home next week because I won't need to differentiate between the cars I tuned (mostly "can tune", really) to the max just on parts that can be bought right now and the cars that can be tuned with "S" parts, especially those that can only take the S parts that can be removed.
I have mine, and last I checked these are the only 41 cars that have WR4 available:
AMG Mercedes-AMG GT Black Series 2020
AMG Mercedes-AMG GT R 2017
Aston Martin One-77 2011
Bugatti Chiron 2016
Bugatti Veyron 16.4 2013
Chevrolet Camaro ZL1 1LE Package 2018
Chevrolet Corvette C7 ZR1 2019
Dodge Charger SRT Hellcat 2015
Dodge Charger SRT Hellcat Safety Car
Dodge Viper GTS 2013
Ferrari Enzo Ferrari 2002
Ferrari F40 1992
Ferrari F430 2006
Ferrari F50 1995
Ferrari F8 Tributo 2019
Ford GT 2006
Ford GT 2017
Honda NSX 2017
Jaguar F-type R 2014
Lamborghini Aventador LP700-4 2011
Lamborghini Aventador LP750-4 SV 2015
Lamborghini Murciélago LP 640 2009
Lamborghini Veneno 2014
Lexus LC500 2017
Maserati GranTurismo S 2008
Mazda RX-7 Spirit R Type A (FD) 2002
Mercedes SLR McLaren 2009
Nissan GT-R 2017
Nissan GT-R NISMO 2017
Nissan GT-R Safety Car
Nissan R32 GT-R V-Spec II 1994
Nissan R33 GT-R V-Spec 1997
Nissan R34 GT-R V-Spec II Nur 2002
Porsche 911 GT3 RS (991) 2016
Porsche 918 Spyder 2013
Porsche Carrera GT 2004
Porsche Taycan Turbo S 2019
Tesla Model S Signature Performance 2012
Toyota GR Supra RZ 2019
Toyota GR Supra RZ 2020
Toyota Supra RZ 1997
Only some these cars have WR5 available. No car with only WR3 has a WR5 available (for obvious reasons).
And yes, this is going to give me extra work on the spreadsheet since I'll have to add these new parts alongside the engine swaps.