No laptop can run Virtual Instruments with low latency on their own due to crap internal sound cards, however there are many external USB/Fire wire (Check to see what ports are available on the laptop) Audio/Midi interfaces available which will solve this problem. (Just do a quick Google search)

The rest is pretty much the same as a PC (Massive graphics power has little relevance with virtual instruments) so go for a spec that meets the requirement for Kontakt 4.

NOTE: It is unlikely that a laptop will have a HDD faster than 5400 rpm as standard, so you may need to upgrade it to a 7200 rpm drive (Their pretty cheap these days) if you have problems.

As you already have a computer, then providing it is in the same room, you can also use this to rum Kontakt 4. (Again you may need to upgrade your sound card)

Hope this helps

Bill