From memory, I think the Mk 4 had more lands and grooves, plus there were improvments to the trigger mechanism and the loading system. While it was an easier rifle to make (and therefor faster into the hands of soldiers) it was also more reliable.
I have both but as the eyes get older I find the No 4 is my choice for Bisley Civilian Service Rifle competitons, although the SMLE will always represent the best battle rifle that this country has ever had.
The Lee action came into service in 1888 with the Lee Metford and if I am not mistaken the L42 sniper rifle entered service in 1970, not a bad reign eh?
The No. 4 is better, being a product improved version of the SMLE. In comparison to the SMLE, the No. 4 has:
- a heavier barrel that is more accurate and better suited for rapid fire;
- a longer sight base;
- aperture sights;
- a stronger receiver;
- a lighter, simplified nose cap;
- better stock bedding, giving improved accuracy;
- and most importantly, it was designed from the outset for mass production;
To correct an earlier post, it isn't true that the No. 4 had more lands and grooves than the SMLE. As a wartime expedient, many US and Canadian made No. 4s were produced with the cheaper 2-groove Mk2 barrel, which tests showed were just as accurate as the Mk1 with 5 grooves.
No4 is instrinsically more accurate and easier for a novice to shoot more accurately (stiffer receiver, heavier barrel, aperture sights, simplified bedding, etc), but for actual combat use - short range rapid fire, snap shooting, close country, etc - the SMLE may well have the edge in many circumstances, as it has much quicker shotgun-like sight acquisition.
The Aussies of course carried on using the SMLE through Korea and until the late 1950s, with no perceived need to switch to the No4. India carried on producing the SMLE - in .303 and 7.62mm versions - until the late 1980s.
Apart from anything else, you can fix 18" of bayonet on the SMLE, as opposed to 8" on the No4, so it gets my vote. Anecdotally, the SMLE was the preferred choice of WW2 veterans; this may have been down to build quality, or the "Legend" of the SMLE.
I own an SMLE (deactivated, sadly) and Ive fired the No4 a few times. I do like the quick sight picture on the SMLE, as I found the no4 rather hard to bring to bear in a hurry. Themanwho's sentiments about the bayonet are also sound; I have one for my SMLE and it is a truly nasty piece of work. According to grandpa, who has now sadly passed away but relayed many a warry tale in his time, the SMLE was his own personal choice at Monte Cassino because it was better at shorter ranges because of the sights and (this is typical of my family) you could use it as a club without worrying about damaging the sights.