Android crunching is a little tricky at this point because it's still largely new... If you run 2/4 cores, you are less likely to have errors and those 2 WUs will finish faster. However, they won't finish with enough time savings to outpace 4 simultaneous WUs. If you're not having an unusually high error rate (seems nearly everyone has some errors on Android) then I'd probably leave it running on all 4 unless there is another reason you need that RAM. I know on my old dual-core tablet (with only 512 MB of RAM) that even having BOINC tasks loaded would really slow things down.