One test, with four parts to it, covering the four areas. compare: "...there will be a test of reading, of speaking, of writing, and of listening..." = more likely to be 4 tests.
The speaker should add words to make this issue clearer, such as: "...there will be separate tests covering/of reading, speaking, writing and listening..."
Further to the above: "...there will be a test covering reading, speaking, writing and listening..." makes it clearer that students will sit one test covering those 4 areas.
Read what I wrote carefully; and note how I rephrased your original sentence, and the possible change in meaning.
There can't be "zero possibility that we will sit four tests": there is "..a test..." = one definite test at least is planned. The problem is whether that test covers the four areas, or the speaker means a separate test of each area.
I was a bit confused because your first answer, "One test, with four parts to it, covering the four areas", sounded very straight and clear to me.
I understand now that it is still not clear how many tests we're going to take at this point, and if the teacher's already decided it and wants to make it clear, he needs to change or rephrase the sentence.