A box that is 50 cm x 60 cm x 100 is approx. 19" x 24" x 39". In a box that size, I would only do 1 or 2 small plants, like Lesso says. You do not have room for 4 plants. Putting too many plants into too small a space is counterproductive and you will actually get less than if you put less plants in. In other words, more plants does not mean more bud. Whether you can get 1 or 2 in there will depend on the strain you grow. No, do not use the multi-plant box you made. Like Rosebud says, do not put more than 1 plant into a pot.
When you grow from seed, you really need/should wait until the plants are sexually mature to flower. I also have real concerns about 100cm being tall enough to really grow anything. The light will take up at least 9", the pot will take up about the same or a bit more. You need space between the light and the canopy. This leaves very little space for the plant to grow. You are going to need to be very careful about the strain you get and you are going to have to do some real training. Is there any chance you can make this space bigger....it is really so hard to grow in confines like this.
Rosebud, I just ask my phone to convert the measurements to inches--couldn't be simpler. 100 centimeters is a meter, just slightly bigger than a yard at 39.37". The metric system is actually so much simpler. I don't know who thought that having a number system based on 10 and measurement systems based on nothing constant was a good idea....