There are two things going on here. One is your connection to the Internet, which is limited to 4Mbps or less. The second is the connections within your wireless network, between multiple PCs (desktops and notebooks) and peripherals such as wireless printers and servers. Having faster networking equipment

(routers and adapters) won't affect how quickly you can upload and download files to and from the Internet--your cable provider puts the cap on that. But if you have a home network with multiple PCs and you transfer a lot of files between them (say, transferring music or video files from a desktop to a notebook), a faster router/adapter combo can shorten the time it takes to make these transfers within your network. Also, if you have a larger home or a house with lots of structural interference, a MIMO router can help push your signal to the far reaches.

Also, keep in mind that the bottleneck on your network will be the slowest component. That is, if your laptop's adapter maxes out at 54Mbps, you won't get the fast transfer rates promised by a 108Mbps or 240Mbps router, because your adapter can't support those high rates.