The Vanguard Up-Rise II 43 is both a large and affordable sling bag. While most sling bags don't hold much gear, the Up-Rise II 43 will accommodate a DSLR with a battery grip attached as well as up to four extra lenses and a flash. The main compartment is well organized with customizable dividers--you can use the top as a large spot for accessories or adjust to fit more gear. Besides the large capacity, the bag is durable and well-built. A side zipper allows for quick access without taking the pack off, while a stabilizing strap helps prevent the bag from slinging around when you don't want it to.
While the bag will fit plenty of gear, it won't accommodate a laptop or a tripod. But, if you are looking for the quick access of a sling bag but want to stow away four lenses, the Up-Rise II is a solid option. Available for around $100, it's competitively priced too.