The Gourmet system inserts are about 11" diameter, about like a "regular" stove frypan.
The Brazillian Ice is smaller, 10x7".
The Kettle Q is largest, about 17" diameter.
I should think the Weber griddle could handle 4 burgers OK, the Brazilian Ice ... no. The Kettle Q of course has plenty of room but is overkill for "4 burgers" and you won't be using the grill for anything else at the time, like corn on the cob or other veggies that will turn out amazing.
Much also depends on what you like to cook on. If I'm going to the trouble and expense of a "gadget" I tend to prefer cast iron cooking surfaces. So if the Gourmet insert griddle is too small there are plenty of rectangular griddles around from various companies, Weber, Lodge, Pampered Chef etc.
If you're a round-griller ONLY type of guy and want an efficiently-sized round SS surface, the Kettle Q makes sense if you dedicate it to the task. Keep in mind you give up more of the Weber charcoal benefits such as the natural smoke flavor (because it isolates food from charcoal) and ability to cook above and below the 22.5" surface while the burgers cook.