One must go cautiously here with increasing light. The plants in the photo appear to be Anubias
species [not sure about the one in the centre planted in the gravel]. Anubias
are low light plants, and very prone to developing brush algae in direct light. They do best in shade, such as provided by floating plants. So when the light is increased (duration) I would suggest adding some floating plants like those I and others mentioned earlier.
Six hours is minimum for most planted aquaria. Depending upon plant species you add, increasing the light to 6-8 hours will probably be sufficient, at least initially. The other thing is nutrients, without sufficient nutrients to balance the light you will have issues with algae. Do you have any fertilizers? I can expand on this later, or you can check out some info here: A Basic Approach to the Natural Planted Aquarium--Part One
The substrate change would make a big difference to the fish and to you viewing the aquarium. I realize the Discus
probably swim about now and then, but it is not simply co-incidence that in the photo they are in the darkest areas of the tank. You can read some background on light and its effect on fish here: Lighting: How It Affects Freshwater Fish