James Dugan, in The great iron ship. New York: Harper, 1953, writes:
'At Paddington, the Great Western London terminal, Brunel kept a private flat car to accommodate his fast horse-drawn britska,which was popularly known as The Flying Hearse. The carriage contained a full-length bed, or at least one that would accommodate his five-foot four-inch person during the four hours a day he lost in sleep.'
I thought I had seen another source that suggested he was not even that tall, but my memory and notes fail me and I can't re-locate the citation. Perhaps, until I do, the 'under' in 'under five foot and four' should be regarded as poetic licence.
John Henderson, Ithaca College Library, firstname.lastname@example.org