Think this way... your mixer is 'submixing' your drum tracks into a stereo track (most likely) before being sent to your laptop. Your bottleneck, therefore, is your recording interface, most likely the laptop's line input, right?
What you need is a multi input interface so that you can connect the drum mics to an individual input that your software will recognize. The hard part is that you need an interface with at least 4 mic inputs and the more mic preamps an interface has, the more expensive. A quicker alternative would be to get a firewire or usb2 mixer; each channel can be recorded to the software as an individual track.
hth,