re being the BJCM expert :) Thanks Tiddly!! 
I have had both the BJCM3 and 4 and I swapped to the 4 specifically after finding the 3 a bit tippy - not much but just at certain times - for example if you can picture pushing the pushchair up to the car door forwards, lifting the baby on in the car seat or into the carrycot, and then you need to do a sharpish turn away from the car to close the door and set off. Thats one occasion where I found the pushchair tippy. Ditto if I went to put something in the car boot and then turned away. So it was more sharp turns with me that caused a problem rather than the pushchair feeling tippy in general IYKIM. Although I did notice it a bit with kerbs as well, and also pushing one handed with the car seat on, on uneven ground eg unmade car parks.
IMHO if you are buying for use with a newborn with a carrycot and / or car seat, then get the BJCM4. For an older child it really doesn't matter.
However I have got the BJCMGT here at present and that does feel more grounded. I ought to try it with the car seat on (makes mental note to do so tomorrow). I do think, that the GT is worth looking at especially buying from birth to last until the end of pushchair days. It just has the extra comfort in the ride, the adjustable handle, the extra PE board in the seat base, the handbrake, the better basket access - I can get a large handbag through the back or the sides of the basket because its elasticated - I got tons in it today - the high sides stop things falling out.
BUT if you have time, and have the money, I would hold out for the Versa - its really will be the Nirvana of pushchairs, and the Versa GT is even better. Having had most of the Baby Joggers now, the Versa is really the merging of all the best bits in one - and there are a lot of best bits to have. Having had the BJCMGT, I can't wait to see the Versa GT.
Just to add, I wouldn't worry about redress - yes JL have fab service and I recommend them, but Kooltrade the UK Baby Jogger distributors also have a fab reputation for customer service, spare parts, generally being lovely. The Baby Joggers come with a lifetime warranty on the chassis, so you can buy with peace of mind. If anything does need replacing long term eg wheels, fabrics etc. then you can buy the necessary parts.