@furēku You are very welcome - and thanks for taking my post in good spirit. It's always disappointing when someone says 'don't do that .....'
Aha !!! You didn't mention that small important part
![Smile [smile]](https://www.festoolownersgroup.com/Smileys/default/smile.gif)
For a secondary internal frame where the glass is a little bit lighter, a 44-50mm material thickness and multiple 10 x 50 dominos as suggested by
@ChuckS will be totally OK - but I still wouldn't use plywood. Spend a little extra and at least use decent-quality solid softwood - I can promise you that it's money well-spent. Do you have sets of long sash clamps for frame assembly? If not - flood the joints with plenty of glue, then put a screw into the joint to clamp it together super-tight whilst the glue dries. Make absolutely sure that your screw passes through the gap between your dominos and doesn't touch the dominos themselves. Drive the head of the screw a few mm below the surface of the timber, and leave it in there for reinforcement, filling and sanding as appropriate to conceal the hole made by the screw head. For your job, I'd go with a 3.5mm pilot hole and a 5 x 80 screw per joint.
Regarding glass thickness - I'd ideally go with 6mm for something this size, but 4mm will be OK. Not ideal - but OK. Make sure it's solidly bedded onto the frame with the correct silicone across its entire periphery - I'd recommend Dow Corning 799, which sets solidly enough to act as a partial reinforcement adhesive, and this will help to compensate for any thermal expansion/contraction of your window. Use 2 coats of good quality acrylic primer and 2 top coats of finish - and fully-finish the frames before you install the glass.
In the UK, our regulations stipulate that any piece of glass located below 800mm from the floor height must be tempered/toughened for safety - so check your local regulations, too. Just a few more questions so I can help you get fixed up:
1 Are the casements going to be hung onto the frames using butt hinges?
2 How are you planning to fix the frames to your walls?
3 What about seals against draughts/cold etc?
Finally - update your profile to show which country you are from. It helps other forum members to give you advice using materials and components which are available locally to you.
Best wishes
Kevin