If we assume each weight must be different...

calc_with_combination.rb

def calculate_combination(num_elements, limit)
  combos = SortedSet.new
  base_array = (1..limit).to_a
  base_array.combination(num_elements).each do | group |
    combos.add(group.sort) if group.inject(0, :+)  == limit
  end
  combos
end

combos = calculate_combination(4, 40)
combos.each { |weight_combo|
  weight_combo.each { |val| print "#{val} " }
  print "\n"
}
puts combos.size