Introduced in August, 1937 (and shown in the middle of this page displaying a number of Portass models) the "4-inch Heavy Duty" Portass was listed as a range of ten models designated Mark 6 to Mark 15. The only difference between the versions was the capacity between centres and their level of standard equipment  - the machines being, rather oddly, being given bed lengths out of sequence with the model numbers i.e. the Mark 6 to Mark 8 took work up to 21 inches long; the Mark 9 and Mark 10 18 inches; Mark 11 to Mark 13 inches and Marks 14 and 15 25 inches.
Distinguished by numerous features, all of which were at variance with normal Portass practice, the "4-inch" could be had with a choice of specification, the makers offering the lathe with or without backgear, screwcutting, compound slide rest and tailstock set-over. Prices varied from the Mark 6 (as a basic short-bed, plain-turning type bereft of backgear, set-over tailstock and with a single swivelling toolslide) at £5 : 19s 6d to a fully-equipped Mark 15 £12 : 19s : 6d. A countershaft was £1 : 19s 6d and a faceplate an additional 15 shillings. The most unusual part of the lathe - and not just for this particular maker - was a changewheel arm carried on the end of a long shaft supported by a bracket extending from the end of the left-hand leadscrew hanger bearing. At its lower end this assembly was fitted with a dog clutch and, outboard of that, a single-slot arm to carry the gears. As well as the inconvenience of making the lathe much longer than necessary, the writer can see absolutely no benefit in this arrangement - and is puzzled as to why it was used. Although the makers claimed that it had the advantage of adaptability for almost any size of wheels (gears) for cutting a big variety of threads from coarse to exceedingly fine pitches the single-slot changewheel arm (instead of a forked type) would have only made life even more difficult when setting up, for example, a fine-feed compound train
For a Portass, the cross-slide ways were unusually wide and the compound slide rest handwheels spoked and of relatively large diameter (though the maker's early illustrations showed a much cheaper, 2-bar type). Another alteration from the publicity pictures was the number of top-slide gib-strip adjustment screws: the catalogue promised four, in reality, as with many other Portass models (except the larger Dreadnaught type), the customer received only three. The standard toolpost was listed as an "American type", with a dished washer that allowed the toolholder to be rocked slightly to get its setting correct. The bed too was unusual with relatively thin walls, slotted feet, lack of a gap and enormously deep recesses on the front and rear faces. The front section had its end faces bored through to carry the 3/4" diameter 8 t.p.i. leadscrew that ran though a full nut on the underside of the saddle. Unfortunately no rack was fitted, thus condemning the operator to much twirling of the leadscrew handwheel when the position of the carriage needed resetting over other than a small range.
With the ability to take a 1.25-inch wide flat belt, the headstock pulley was especially robust and was carried by a spindle with a No. 2 Morse taper nose and bored through 5/8" to clear a 9/16" bar.  The tailstock quite unlike anything fitted to another Portass, , while the backgears and output gear on the end of the main spindle were of a much finer pitch than normally used by the maker.
In contrast to the usual tiny Portass advertisements carried by the Model Engineering press, the 4-inch was heavily promoted with expensive front-cover and half-page spreads appearing regularly throughout the second-half of 1937. However, all was in vain, for the numbers sold must have been tiny; today the lathe is very rare - in contrast to the common S-Type and its derivatives.
If any reader has a Portass 4-inch, the writer would be interested to hear from you..