Not much point in mentioning transmissions types/ratios without reference to the differential ratios they were paired with.
In the markets where a 5-speed overdrive type transmission was standard equipment, a 4-speed was offered as a 'showroom option'. It would require a diff ratio change to make it work as intended. However, I very much doubt that such options were taken up in practice except perhaps in Japan, where the selling dealers had a more realistic chance of fulfilling such an order, but then Japanese buyers had the choice of both 4-speed and 5-speed equipped models on the showroom floor anyway. In contrast, here in the UK it was a case of choosing from whatever had actually turned up on the boat... ("do you want the red one or the blue one...?"). The 4-speed was theoretically an option, but I can't imagine why anyone would select that option. They would have been better off buying a Cedric or a Laurel.
When I have asked the "why 4-speed + 3.3:1 diff for north America" question in Japan - including putting the question to Chief Chassis Engineer Mr Hitoshi Uemura - the answer was usually that the driving style of the average north American market customer was perceived as being less 'sporting' than those in other markets, and that a wide ratio 4-speed transmission with a tall differential ratio would be more suitable for the vast majority of such buyers. Cost would also have been a major factor in planning. There are other details on early cars which show the initial cost-cutting focused planning for the north American market Export models. So it was likely a combination of both.
Without wishing to insult, reading between the lines it seems clear that the USA mass market in particular was seen as being less sophisticated, less inclined to live with more frequent gear changes and higher RPM cruising speeds, and that the arch enthusiasts who did require that type of sporting character would be happy to modify their cars to suit (and it appears they did). All the same, it would have been nice to see a more sports-focused model available alongside the 'standard' north American Export model. Something along the lines of Car and Driver Magazine's 'Omega Z', perhaps...?