If there is too little amperage it could damage the laptop. It might cut off prematurely and possible corrupt the Operating System or possible cause damage to the hardware.

Your laptop will work fine as long as there are enough amps provided by the AC Adapter. So, if your laptop requires 4 amps and you use a 6 amp power supply you are cool.

The amperage rating of the AC Adapter tells how much current can be supplied, the laptop determines how much is actually drawn. You don't know you have all that power until you actually call on it. The voltage should be rated fairly close, but the amps don't matter as long is they are equal to or greater.