The biggest difference between them is that the A6 has virtual wall function, and it comes with a virtual wall whereas the A4S does not support virtual wall.
And the A6 uses tempered glass top cover. It is a bit heavier than the A4S.
The A6 has smaller dust bin capacity. Its capacity is about 0.3L whereas the A4S is over 0.45L.
The A6 uses rubber main brush whereas the A4S uses bristle main brush.
Running Time. The A6 runs up to 160 minutes whereas the A4S runs up to 140 minutes.
The A6 uses new Gen 2 Motor. The Gen 2 motor can generate up to 10X the air power.
The main color of A6 is black whereas the A4S is grey.
Both of them come with remote control, feature 2 side brushes and HEPA filter, schedule function, auto adjust between floor types, auto docks and recharges when battery is low, etc.
You can download iLife A4S and A6 manuals to learn more.
A4S guide – http://www.iliferobot.com/list-98.html
A6 guide – http://www.iliferobot.com/list-99.html
Any other difference you know? Please share below.