I upgraded our low beams to H9 halogens, they are MUCH brighter than stock. The upgrade is super simple and costs less than $20 (Amazon usually has them for less that $10 per bulb).
Is there any modification involved, or are these plug and play? What's the difference between these and the regular halogens?
The difference is that the H9 bulbs put out about twice the light as the stock H11s (this mod is only for '06+ projector headlights). Best under $20 mod that I've done to our 4Runners.
To get them to fit, you need to break off a small plastic tab from inside the plug area of the H9 bulb (some cars require a metal tab on the bulb to be trimmed, but they fit both of our 4Runners without trimming). No modifications are needed to the headlight housing.
Here's a decent writeup on the H11 to H9 conversion with pictures. I used something very similar to the sidecutters mentioned on that page.
you say only for the '06+ headlamps. I am curious why cant this H9 mod be done on the '03-'05 4Runner?
If you own a 03-05 and don't want to glare all over,, look at the HIR bulbs (not HID). You need to mod the base slightly (30 sec) and then you have more light output that works with a reflector.
If you want to do HID do it right by retrofitting a projector. Here is a link: http://www.toyota-4runner.org/4th-gen-t4rs/183555-tins-mini-h1-retrofit.html