Go and buy two pairs this time. Chances are that boot will no longer be made in 2 years let alone 4 in the same way you like. If they are comfortable and they lasted you that long with no issue those are good boots.
I'm a shoe cobbler as the username might suggest and most modern shoes have a lifespan of about 1-2 years depending so getting 4 with daily use is kinda nuts.
As for the heel kicking out I assume you mean the outer back of the heel is worn down more than the inside back yeah? In that case that's totally normal and its just because that's the first part that hits the ground when you take a step. If its uncomfortable then sure you can get something done about it but it generally shouldn't unless its really worn down, like so much that you'd be capturing rocks in the holes in your sole. Thats also a pretty easy job for a shoe repairman to fix that'll be cheaper than a new set in most cases