The next-generation 4Runner will undoubtedly get an mpg upgrade, but I wouldn't get hopes up on it being anywhere near a Highlander.If the 2023 Toyota 4Runner finally gets a mpg upgrade, I would love to consider that...
My informed speculation based on the new Tundra announced Sunday is the focus of the 4Runner hybrid will be more on power than mpg. The 4Runner and Tundra (and other body on frame Toyotas) will all be built on the TNGA-F global platform and are engineered by the same team, so it's reasonable to expect the 4Runner will have a similar design.
The new Tundra hybrid is an entirely different design than existing Toyota hybrids. It's a single motor-generator sandwiched between the engine and transmission. The primary purpose of the motor is to add torque. Increased efficiency is a nice additional benefit, but the design precludes it from approaching the efficiency of the Highlander hybrid system.