im not saying that it can't be done. I had DIY co2 on my 50g getting me 25ppm at one time. What I am saying is that you will not be able to maintain the "consistency" that is necessary for a planted tank.
You will have to constantly be shaking up the bottles and remaking the bottles in order to maintain. And you will have to constantly be checking the bubble counts and pH of your tank. DIY is just way too variable. And with algae loving variability, it just isn't worth it.
You could still try it though, I just bet that it will take you 2-4 weeks to say screw it and quit DIY
happened with me anyway... too much time and labor for something that will most likely be unsuccessful anyway.
To help you out with your mixture though, I think you are under-dosing the ingredients. Here is what I used to use on my 40g per 2L bottle (you are using 4L so you will want to double it):
2cups granulized sugar
2t Bakers Yeast (note that this is not Brewers Yeast.. Bakers Yeast is the granulized yeast)
1t Molasses (or protein drink)
1/4t Baking Soda
1-1.5L De-chlorinated water
1) Desolve the 2 cups of Sugar in warm-hot water (de-chlorinated). Either in the 2L bottle, or a separate container (then pour in 2L bottle)
2) Put in the 1 tp of Molasses
3) Fill bottle with room temperature or cool de-chlorinated water to ~3-4" from the top.
4) Put in the Baking soda and Bakers Yeast
5) Shake briefly with cap on.
6) allow time for solution to reach room temperature and it is ready.
7) *sometimes takes 24 hours for co2 production to get sufficient*