Hi 
The most stable version of LO right now is the 3.6.7 but the 4.0.5 is so rock solid that it's doubtful you would need anything stronger than that.
Anyhow it seems you have a deep misunderstanding about the reason for LibreOffice's rapid development.
There is 1 huge difference between OpenSource and proprietary systems with regards to updates, patches and all the rest.
* In proprietary systems almost all updates claim to be "security updates" and often patches are supplied in between times to deal with security issues.
* In OpenSource systems the security tends to be there already. Security is about the top priority. In order to even work at all on Gnu&Linux each program has to be written to be very secure and take notice of crucial issues. The under-laying OS separates out normal users from Admin/SuperUsers to such an extent that programs have to be able to run fully as normal users. There are no easy short-cuts around that.
So, OpenSource updates are usually about adding functionality. Where bugs happen they are seldom about security and rarely make the system vulnerable. It's more likely that some feature simply doesn't work or does something a bit strange. If anyone does spot a weird bit of coding that could potentially be a security issue then it is given a HIGH priority and fast-tracked through to try to fix it before the code is released.
Proprietary tools prioritise features rather than security and no-one gets to see how badly any of the code is so there is no-one to really criticise kludges. It's only AFTER hundreds of thousands of machines are affected by a security breach that companies tend to start thinking about diverting resources into fixing it. Even then they are likely to try to wrap it into their new version and make people buy that rather than give the patch for free.
The aim with proprietary tools is to find ways of forcing people to buy newer versions in order to carry on doing the same work they can already do with older versions.
With OpenSource it's more about the excitement of being able to do new things. No-one really cares if you stick with an older version because no-one makes any profit if you upgrade or not. So you can stick with older versions if you want and many of us do until we just can't stand it anymore and want to join in with the fun everyone else is having with the newer versions.
So, OpenSource updates are seldom about security. They are almost always bug-fixes to increase functionality or else are added functionality that is completely new.
Regards from
Tom 