An accurate diagnosis is going to be difficult without some further information and even some images of the your Monterrey oak.  At this time of year, the Monterrey oak (also know as Mexican white oak) is going through a transition and the old leaves will turn yellow as they are cast from the tree.  This often happens in the top of the tree first and then progresses down to the lower crown as we get into early spring.  However, the new leaves should certainly not be falling off.  I have not kept up with the weather in Bexar county, but a late freeze might be responsible.  Monterrey oaks are not particularly cold hardy.  Also, overwatering could be a problem.  As with most drought hardy species, they do not like "wet feet" and will respond pretty quickly to prolonged soil moisture, particularly when planted on poorly drained soils.  There are other problems, such as girdling roots, or being planted too deep, that could be showing up in the fourth year as well.  As I said, if you would like to provide further information and images, please do so and I will take another look.  I would like to see the trunk, from the soil around the base of the tree up to the first lateral branch.  An image of the whole tree in the landscape would be useful, along with close-up images of the blooms and symptomatic leaves.