Go to F9, the database. go to the 'items' subcategory.
There on the left side you can see the list of all items which are in the game. Click on the 'change maximum' button, and pick 21 (you can increase it more to create new items). Then click on the new empty bar in the items list numbered 21. Give it a name, an icon (double click) and description. (e.g 'Use this piece of timber to build a bridge') Leave all of the rest blank. then go make whatever events that gives you the timber.
THE BRIDGE----
All of this becomes useless unless you do exactly what I say right now =D
http://img360.imageshack.us/my.php?image=picturesl2.png
OK just follow the instructions and look at the screenshot if you don't understand.
First choose a name for your variable. this doesn't really matter, just for your own comfort.
Under operation, choose 'set'. that means it will set the variable at the number of what you choose.
Under Operand, choose item, choose the wanted item (in this case, timber) and... thats it for this comand,
Then make a new event command-conditional branch. at the first page pick variable, the variable you chose (for this instance, the amount of timber). Pick one of equal to, greater or equal to, blablabla (in this case it seems equal to would do)
Pick your number. you wanted 4? 4 it shall be.
After making your conditional branch you'll see theres an else thingy bellow it. Put ABOVE IT what you want to do when player has all timbers. put BELOW IT what you want to happen if player doesn't have 4 timber. What you put after Branch End will happen in both cases, with or without the timber.
This is as detailed as I can go.
If you think I hellped you, please say MUFFINS ROX! instead of a thanx.