How do you replace a 2004 Toyota Highlander air filter?
This information can be found in your owner's manual. If you do not have one, then I suggest you get one. You need an owner's manual to know what items to service and when, and for specifications. You can order one from a dealer, find one at a salvage yard or even eBay
The air filter is mounted right behind the radiator and is secured by 4 clips. Just pop the 4 clips and remove the cover (bringing it forward). My highlander manual didn't have anything, but it's pretty obvious... The cover is about 15" wide and 6" tall. The replacement filter is "keyed" in that one corner is squared and the others are rounded, so you can only install it one way.