Hang about - 0.4 is 40%, not 4%, and by 'concentration is 4%', do you mean that what you are using contains 4% of what you want to get up to 0.1/litre? If so, then shouldn't the calc be:
(0.1 x 200)
_________
4
x 100
= 500?
If not, then "0.04 x .01 x 200 = 0.8