When you said you were looking for a hydrangea that can tolerate full sun, I got confused. Did you not say that the corner gets only four hours of sun? Or are you planning to place a hydrangea elsewhere?

In the South, hydrangeas need protection from the sun. Too much sun and the leaves yellow out or turn white-ish. It is called sun scorch or sun bleach. Hydrangeas also need well draining, moist, acidic soil. When the soil is acidic, colored mopheads bloom blue blooms; otherwise they bloom purple or pink blooms. People who have poor soils (clayish, for example) can get around this by using raised beds although hydrangeas will tolerate imperfect soils up to a certain point. But when soils and alkalinity are pretty bad, I recommend trying to grow them in pots instead. What type of soil do you have?

My hydrangeas here in Texas have not had much problem with the high temperatures in the 100-110s but they will wilt when you add some drying winds so you need to keep an eye for wind advisories and you need to control the urge to water when they plants look wilted but the soil is moist. I give my plants shade starting at around 11am.