попробуй без скобок
$thr=thr1
SetThread($thr, 1)
FOR($count = 0, $count < 4)
SETTHREAD(STRCONCAT(THR_, CHAR(65+$count)), 1)
END_CYC
WHILE($count ! 0)
WAITMS(5)
END_CYC
HALT
THREAD(THR_A, 0)
PRINT("A")
INC($count, -1)
SETTHREAD(THR_A, 0)
END_THREAD
THREAD(THR_B, 0)
PRINT("B")
INC($count, -1)
SETTHREAD(THR_B, 0)
END_THREAD
THREAD(THR_C, 0)
PRINT("C")
INC($count, -1)
SETTHREAD(THR_C, 0)
END_THREAD
THREAD(THR_D, 0)
PRINT("D")
INC($count, -1)
SETTHREAD(THR_D, 0)
END_THREAD