As one can notice the differences are not very significant. The BS6 is a tiny tad down in power and a tiny tad up in weight compared to the BS4 - which makes the BS4 to perform tiny tiny bit better. The BS6 is however described as a bit more refined, especially fuelling and also gets a tiny tad better fuel economy.
My opinion - after riding Royal Enfield Bullets 500 in India, Nepal and through Bhutan and loving those machines I was a bit intrigued when I heard about the introduction of the Himalayan. A Royal Enfield suited for long distance and offroad riding? How awesome and cool!
To make a long story short - I didnt get to test a Himalayan until January this year where I did and extended Mae Hong Son loop with several significant detours and with a pillion.
The conclusion - it was a BS4 edition I rented btw - although not without charm and character the Himalayan will lose to my thrusty «adventurised» Honda Crf250L in 9 out of 10 features and characteristics. And I dont think a BS6 edition would have changed my thoughts.
By all means - the Himalayan either it is a BS4 or a BS6 is good bikes for what they are with character and charm. Its just that there are so many other bikes that are better in most comparisons.
Considering there usually are plenty of BS4 models for sale on Kaidee of Facebook Marketplace for around 100 000 baht or a tad more - I personally would have gone for a second hand low milage BS4 if I ever were to buy one.
Just my 50 satang worth of thoughts though….