Four nights is about right. The Riverwalk area downtown is not really that big and easily negotiable on foot from any Riverwalk hotel.
Best location is probably the Hyatt Regency.
Best full suites with breakfast is Embassy Suites on Houston.
Best Historic luxury hotel is the St. Anthony.
M ost Historic is the Menger.
Best Riverview balconies is La Mansion.
Best budget hotel - Drury Inn and Suites
Best Hilton is Palacio del Rio.
I would avoid the hotels near the convention center as they are nothing special and the poorest location.
With all of that, if you don't mind using a rideshare the best hotel downtown is at The Pearl, The Hotel Emma... that is where I would stay. If on the river we stay at either the Palacio del Rio, Embassy Suites, or La Mansion.