plainglazed

Label the coins 1-8.  There are 56 possible combinations of the Heavy and Light coins.  I think they can be determined in four weighings:

1) Compare 1,2 v 3,4
 If equal then the fake coins are either 1,2  3,4  5,6  5,7  5,8  6,7  6,8  7,8 (16 combinations)
  2) Compare 1,3 v 2,5
   If equal then 1,2,3,4,5 are "normal" (label X)
    3) Compare X v 6
    4) Compare X v 7
   If > then either 1H and 2L or 3H and 4L or 5L and one of 6H,7H,8H
    3) Compare 4 V 5
     If equal then 1H and 2L
     If < then 3H and 4L
     If > then  5L and one of 6H,7H,8H
      4) Compare 6 v 7
   If < then either 1L and 2H or 3L and 4H or 5H and one of 6L,7L,8L
    3) Compare 4 v 5 as above
 If > then the fake coins are either 1H or 2H and one of 3L,4L,5L,6L,7L,8L or 3L or 4L and one of 5H,6H,7H,8H (20 combinations)
  2) Compare 3 v 4
   If equal then 1H or 2H and one of 5L,6L,7L,8L
    3) Compare 1,5 v 2,6
     If equal then either 1H,5L or 2H,6L
      4) Compare 1 v 2
     If > then 1H and one of 6L,7L,8L,
      4) Compare 6 v 7
     If < then 2H and one of 6L,7L,8L
      4) Compare 6 v 7
   If > then 4L and one of 1H,2H,5H,6H,7H,8H
    3) Compare 1,2 v 5,6
     If equal then 8H
     If > then either 1H or 2H
      4) Compare 1 v 2
     If < then either 5H or 6H
      4) Compare 5 v 6
   If < then 3L and one of 1H,2H,5H,6H,7H,8H
    3) Compare 1,2 v 5,6 as above
 If < then the fake coins are either 1L or 2L and one of 3H,4H,5H,6H,7H,8H or 3H or 4H and one of 5L,6L,7L,8L (20 combinations)
  2) Compare 3 v 4 as above

Could have easily messed something up but seems four weighings should somehow work.

• Travel from one side of a curved path to the other -cross arc
• “I had to write an IOU
  / To all the playas in my crew
  / No drinkin’ Chivas in bed
  / Because I’m too in the red”, e.g. -debtor rap
• Buddy to play Twister with -tangle pal
• Period of Turkish rule -Ottoman era
• Shout from the ground to someone on a balcony -yell upward
• Place to get a rotini martini -pasta bar
• Stomach problem for a stressed-out La Paz resident -Andes ulcer
• Apprehension of a weasel -ferret nab
• Meshed containers for edible bulbs -garlic nets
• Agent of darkness -night rep
• Make a sketch of part of a block and tackle -draw pulley
• Rude, unrefined member of an ugly, fanged humanoid race -crass orc
• Someone who computes unions -set adder
• “Watch out for the Apple computer” -beware mac
• Fanatical collector of old tunics bearing coats of arms -tabard nut
• Rodent willing to die for its beliefs -martyr rat  
• Distilled drink from southwest Australia -Perth gin
• Where a colorful pet bird might sleep -parrot bed (although don't birds sleep standing up?)
• Genetic information of a person who lives in isolation -recluse DNA
• Cloth used to clean a signmaker’s template -stencil rag
• Flying mammal native to far northern treeless regions -tundra bat
• British streetcar covered with a dark viscid substance -tarry tram
• Sports stadium slogan -arena motto
• Official supervising playful conversation -banter ref
• Small, biting fly that has landed near one’s boutonniere -lapel gnat
• Weekends and holidays on a calendar -red dates
• Plant fluid obtained from Morocco -rabat sap
• Interconnected network of photo equipment -camera web