I dont want to be negative as I now realise you have made a serious investment with you tank however I will tell you what I honestly think!
There are several reasons why toughened glass is not generally used in fish tank construction.
It is extremely brittle, there is no warning cracks or leaks just an explosion as the tank shattered into millions of pieces.
Even the slightest irregularity on the base can cause the tank to explode as there are no flex properties in the glass so a thick rubber or high density foam base is essential.
You can not drill or cut the glass.
At 4 foot tall tank could see 1000 lbs on the bottom glass section and over 740 PSI on the sides near the bottom, at the bottom seams the water weight would be trying push the seams apart, I would not personally trust silicone to hold this on its own.
Toughened glass is stronger than plate glass but, I cant find any calculations for pressure rating it either!
I was going to suggest that you used the tank as paludarium & only half fill it with water however you would really need a door to gain easy access & this would mean taking one of the glass panels out & fitting some sort of upper half hinged glass door?
Sorry if I have given you caurse for concern & it may well be a successful & water tight tank?