The normal way would be to build a square frame 3.8 meter by 1.4 meter out of wood framing, then add 4 more 3.8 meter pieces spaced equally between the 2 long sides. Then add a plywood top. Because it is only about 150mm thick and built of wood it will sag and be bouncy. It will feel weak, but it will not break.
Next, get a 1.4 meter long piece of steel that is 150mm wide and about 10mm thick. Drill holes in it and screw it into the 1.4 meter piece that is closest to the outer frame, the one that is about 1.2 meters from the wall. Then, climb up and see how bouncy it is, and how much it sags. if it feels strong enough, fine. If not, add another piece of steel.
I am leaving out some details, but this is the general idea. You are using the very stiff and strong steel to overcome the wood, which will tend to bend.
In the US, we say that the steel piece has become the 'sister' to the wood piece it is screwed to, that you have 'sistered' the steel to the wood.
I hope that this helps you,