I have a plane consisting of 4 squares - take as an example a subdivided plane. How can I merge these faces into one?
There is an operator called Dissolve which does exactly what you want.
Select everything, with A, and then press X (or Delete) and select
Limited dissolve. This will do the same thing as above, but will not require you to select the proper vertices manually.
so long as the faces are surrounded by more geometry, using f for Make Edge/Face works well. For a whole simple sub-divided plane like the example given, this tool will make a mess.