Here's what they had to say about the 4-pocket tables (edited slightly for clarity):
Mr. Kearney worked on this D. Harris snooker table in the 1980s. It was 8' x 4' and only has corner pockets! Quite how anyone plays any form of Billiards on this table is something of an enigma. Peter Ainsworth, a notable snooker historian had researched this question in the past. In his research, he became aware of some American 4-pocket billiard tables being produced. He noted that some of the American snooker tables were made in the period from 1863 to 1869, at a size of 6' x 12' with only four pockets, followed by some made from 1869 to 1873 at sizes of 5 ½' x 11', again with only 4 side pockets. Then from about 1873, full carom tables (without any pockets) were being made at 5' x 10'. So from this, Peter Ainsworth feels that the 4-pocket D. Harris snooker tables must be pre-1873 when the fashion for 4 pocket tables died out in America.