How should I build a wooden gate?
In an attempt to answer some of the questions raised in your posting let me respond in answer format. 

1) It can be advantageous in some situations to affix a timber post to the brick or concrete posts/walls in either side of the gate opening. This post can then serve as a better medium into which to attach hinges and latches. 

2) A 3.8 meter wide gate will develop a huge amount of lever arm out at the latch end of the gate and this needs to be taken into account when developing the gate design. 

3) The size of wood used can play a big part in how heavy the gate will end up being. The type of wood can play a role as well. This is one reason that boards are often used for gate facing instead of thicker planks. Cedar wood is also much lighter than hard wood like oak and thus becomes a popular choice for gates. 

4) Adding appropriate bracing into the design of the gate will keep the gate from sagging. 

5) Successful wide gates can often benefit from making the aforementioned side posts much higher than the gate opening and then using a cable from the top down to the end of the gate away from the hinges to help support the weight of the gate. Sometimes you can see the tops of the two higher posts connected across by a horizontal member. This provides support from post to post to keep the weight of the gates from pulling the posts to the sides.