What is the meaning of asool e arba?

It's called Usul-e-Arba. This is the title of an academic assignment written by Salam-u Alaykum the topic of which is Taruf e-Hadith, or an Introduction to Prophetic Tradition. It's subject focuses upon Islamiyat and gives an introduction to the importance, history, content summaries and ategories of the first 6 books of Sunni Islam, as well as those of the first 4 books of Shia Islam.