Are four brackets adequate for an 8-foot wire laundry shelf?
I would say it all depends on how much weight you plan to put on the shelves. 

Here are some "specs" on a site for Closet Maid Maximun hold 6' x 16" shelving.

Holds up to 600 lb. per shelf (total weight per 40 in. track not to exceed 900 lb.)  

Weight should be distributed evenly across shelf.  

Can be used with maximum load installation hardware (sold separately)

Installation in studs or concrete with standards and brackets (sold separately) no more than 16 in. apart

http://www.homedepot.com/p/ClosetMaid-Maximum-Load-6-ft-x-16-in-Ventilated-Wire-Shelf-73571/202104723