For future reference, please see the "Pathfinder/QX4 General Information and FAQ" thread pinned at the top of the Topic List.
From said thread:
Wheels/Tires:
Q: What is the lug pattern?
A: 6 x 139.7mm (6 x 5.5in) for WD21s and R50s, 6 x 114.3mm (6 x 4.5in) for R51s.
Q: What is the center bore?
A: 100mm (3.93in)
Q: What is the largest tire I can run with factory suspension?
A: It is not recommended to use a tire taller than 31" as it can cause rubbing on the fender flares or front bumper. You may be able to fit larger tires, however that would require removal of the fender flares as well as an aftermarket bumper (unless you cut away some of the front bumper) and other various trimming/bending.
Q: What is the largest tire I can run with the A.C. (Automotive Customizers) lift?
A: With some serious trimming and bending you may be able to fit as large as a 34" tire, given the fender flares are removed and an aftermarket front bumper is used (unless you cut away some of the front bumper). The A.C. lift has different outcomes on different vehicles, some net more than 2" of lift (after settling) which can allow for larger tires.
Q: Which are better, steel or aluminum wheels?
A: It depends on your application. Steel is much stronger and much cheaper, however it can rust very easily and it weighs significantly more than aluminum or alloy wheels. Aluminum (or alloy) wheels are much lighter and will never rust but they can be quite expensive. If the vehicle spends a lot of time off-road, then steel is recommended for strength. If the vehicle is a daily driver or spends most of its time on the road, then aluminum (or alloy) is recommended for weight savings and better fuel economy. Either wheel will perform the same in either on-road or off-road applications; it really comes down to personal preference.