plants, even though low light, consume the Gh?
When plants are growing well they do consume calcium and magnesium which are detected by the GH test. The KH test detects carbonate ions C03. However CO3 doesn't exists by itself it is always attached to something else. Most of the KH in tap water is in the form of calcium carbonate and magnesium carbonate. Plants will consume the carbonate and magnesium and calcium.
In the case of the Sachem Alkaline there website indicates it is based on sodium bicarbonate chemistry but the information is limited on their web site. If it is then the plants could consumthe carbonate and then leave the sodium in the water. This would lower KH and have no effect on GH.
So yes plants can reduce GH and KH. however, 4 degrees is equal to 71ppm. Given enough light, CO2, nutrients (other than calcium and magnesium) and time plants can do this. But it is highly doubtful that your plants can do this in 4 days in low light, excel, and root tabs. There are simply too many limitations in your aquarium for this to happen in 4 days.
My guess there is some sort of reaction going on between Alkaline and Excel. Or maybe a reaction between them and stratum. I would try discontinuing the Excel to see what happens. If you have enough water flow and agitation of the water surface you shouldn't need excel in a low light tank (there is a lot of CO2 in the air).
If the GH consumption drops by just removing excel then it might not be able to use it in your tank. If it has no effect and the Gh and Kh drop continues then it must be a reaction between the plants, substrate, and equilibrium and Alkaline. I don't know anything about stratum but if it is a buffering substrate you might want to then try eliminating the Alkaline for a while. Plant don't need the buffering effect of KH so it should be safe to do this before fish are added. Fish are more strongly affected by sudden PH swings and they do need some water KH.
I personally don't like Excel. When dosed correctly it will kill some plants. most others are not affect. But if dosed incorrectly it can tell everything, bacteria, fish, plants, in the tank. On some people it can cause skin irritation. It is very corrosive. In a low light tank with sufficient aeration you don't need it.
Also is there a reason why you are trying to keep your GH and KH at 4? In my medium low tech tank filled with 100% RO water I have only found that I need 2 degrees GH for shrimp to properly molt and it is more than enough for my plants to grow without any calcium magnesium deficiencies visible. Also I have a few old nail shells in the tank. These shells are primarily calcium carbonate. and they keep my PH avery close to 7 and they will easily last years before eroding away. My substrate is quartz gravel (inert). And the gradual consumption of my GH booster does very slowly increase KH. I personally don't see the need to continuously buy excel when CO2 is free, and adjust KH when a couple of snail shells costing almost nothing will last years.