Water going to clear?

Is the hang on filter new? Did you do anything to cycle the filter? What are your water parameters, such as ammonia, nitrite and nitrates? What ph level do you consider "good"?

From what I understand yellowish water is caused by DOCs (Dissolved Organic Compounds) and is present from a variety of different situations. Usually driftwood is the cause but it can also stem from an algae bloom (although unlikely), abundance of fish waste or various other circumstances. The main concern is that it could effect your PH causing it to change over the course of 24 hours possibly causing harm to your fish. What is your current PH and what is your ammonia, nitrite and nitrate at?

I would recommend daily testing and water changes to make sure your parameters stay in check and maybe look into adding carbon to help pull some of the chemicals out of the water. Another possibility could be you don't have enough filtration to keep up with bio load or mechanical filtration and this could be a factor.

Since you said I just got a standard 4 foot aquarium I'm going to go out on a limb and say its more than likely a parameter problem.

seachem makes a product called clarity. It's a flocculant which means that small particles bond to it, and then aggregate together which makes them bigger and thus easier for your filter to screen out. Might be worth looking into.