Aces are meant to take kings. It is very rarely correct  to underlead an ace against a suit contract.  Not   only may it give away a trick because declarer has the unsupported king without the QJ in dummy , or  your partner may not be able to read the situation. If you lead the four of hearts  against the contract of four spades, and   dummy has has QJ532, your partner with K976 should duck because declarer should  have a singleton ace .   Similarly, partner should play the eight  from  K98    if dummy has Q1043 and declarer plays low.    Again,   If the three has been led  dummy has   KJ42 of hearts  and declarer plays low it is the correct play by your partner has the Q984 to play the eight of hearts  If dummy has the KJ64 and partner has the H92 partner should play the nine of hearts  assuming you led from the heart ten  as opposed to the heart ace .

 

There is one situation in which underleading the ace is often may be considered. That  is when  the dummy on your left has shown a strong balanced hand by opening one or two no trump,  and you feel from the bidding and your hand  you  have no other outstanding lead.