1959 saw the addition of the fourth North Garden Structure on the grounds of the Disneyland Hotel. There were now 234 rooms and suites available at the Disneyland Hotel. As Disneyland grew (Monorail, Matterhorn, Submarines in 1959) so too did the Disneyland Hotel. Although I do not have the exact date of this addition, I do know the structure was built some time between November 1958 and June of 1959. I have a dated photograph showing the building open for business on July 4th, 1959. My guess would be construction started near the first couple of months of the year and was completed by June.
June 1958 Overhead - Disneyland, the Disneyland Hotel (3 North Garden Structures) and the surrounding area in Anaheim
1958 Overhead Close-Up picture - This June 1958 photo shows 3 North Garden Structures.
1959 Overhead - From 1959, showing four North Garden Structures. The below brochure is now listing 234 rooms at the Disneyland Hotel.