That is a lot more mA than the RYOBI batteries I tested and it is at higher voltage.  Should run a drill nicely. What voltage does an unloaded 18 volt battery put out when it is fully charged?  My 14.4 volt LiPo batteries a couple of days after they were charged measured 16.73 Volts.  Using the same ratio would give about 21 volts.  I do not know if that would damage the detector. In my old age I am getting very weight conscious. What does that 18 Volt battery weigh?  The weights are listed on the graphs for the batteries that I tested.
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4 Ah = 4000 mAh 
The lithium rechargeable batteries need to be protected during charge and discharge.  If they are over discharged one time they are destroyed.  I believe all of the drill packs have built in discharge protection.  The charger that come with the drill have charging protection.  Most battery packs we have used have only two leads.  If you look at the Ryobi they have more leads so during charge the voltage across each cell can be monitored and controlled. The discharge protection circuit also monitors the voltage across each cell and if one gets too low it shuts off the drill.
As far as using the 18 V for a detector I like the 4 Ah, don’t know about weight but I do not like the 18 volts.  Quentin’s 12 volt batteries supply power in the voltage range that detectors work well in.