This snippet was taken from www.grayarea.com/sneak.htm and MAY NOT be
posted on your web site. It MUST be linked back to the above URL for 
download.

----[ in fight.c ] ----

in the group_gain() function
add AFFECT_DATA *paf; under CHAR_DATA *gch;
if you dont have a buf2 then add char buf2[MAX_STRING_LENGTH]; under
the AFFECT_DATA *paf you just put in.

somewhere before it gives the player the xp for the kill add:
  
  if ( gch->affected )
  {
      for ( paf = gch->affected; paf != NULL; paf = paf->next )
      {

      if (paf->type && skill_table[paf->type].name && strcmp(skill_table[paf->type].name,"4x")== 0) { xp = xp * paf->modifier;
sprintf(buf2,"{C%d{Wx {MExperience!{x\n\r",paf->modifier);
send_to_char(buf2, gch); }
      }
   }

----[ In magic.c ] ----

void spell_4x( int sn, int level, CHAR_DATA *ch, void *vo,int target )
{
    CHAR_DATA *victim       = (CHAR_DATA *) vo;
    AFFECT_DATA af;

    if ( is_affected( victim, sn ) )
        return;
    af.where     = TO_AFFECTS;
    af.type      = sn;
    af.duration  = 40; /* Feel free to give more or less */
    af.location  = APPLY_NONE;
    af.modifier  = 4;
    af.bitvector = 0;
    affect_to_char( victim, &af );
    send_to_char( "{YYou begin to learn more quickly from your kills!{x\n\r", victim );
    act( "{Y$n{x {Wseems to{x {Gshimmer with knowledge.{x", victim, NULL, NULL, TO_ROOM );
    return;
}

You almost must add this in const.c and set the mana cost etc yourself.
If you alter the name to something besides 4x in const.c you must change
the 4x in quotes in fight.c that you added to the new name.