I've known all the time about the capacitor issue in A4000 (the #1 cause for bad sound or no sound at all in A4000), however I've been told by a friend who was a huge Amiga geek/repair guy back in the day that they simply dry out. Never heard of any leakage.
edit:
So what I am saying is that as long as you've replaced or removed the Ni-Cd battery from your A4000 motherboard you are probably safe from any further damage. The dry capacitors will not cause any harm, but only make your sound dead, and even this won't happen right away but you will notice it slowly degrading through time, until you replace the capacitors with more advanced quality ones.