It's interesting that you say you can't make it beyond 4-5 hours, since apparantly it's just after this (around 6 hours) that apparantly fasting becomes easy (certainly true to my experience). This is when your body moves from the post-prandial stage (after eating), realises there's no food about and so stops demanding a top-up and starts simply burning it's own stores (with lypolysis increasing throughout the fasting period. It's possible therefore, that you simply need to break this wall and then you'll be fine fasting after that.

In my own experience, there are some days/weeks when I can't fast at all and others when it's easy as anything. There doesn't seem to be any easily discernible pattern to this, although amounts eaten and stored previously would be an obvious one, as would exercise done recently. If I've not slept properly, then fasting often becomes impossible.

What do you eat when you're not fasting? If you're eating a decent amount of carb on your paleo diet, sufficient to be running your metabolism on glucose, then fasting will necessarily be somewhat harder, since you'll be switching from a glucose-based to a fat-based metabolism and keto-adapting takes a while. If you're eating a fully low carb diet then you shouldn't have 'low blood sugar' at all, in that you should mostly be fuelled by fatty acids, not sugar. The same will apply if your diet is quite heavily protein based, since above 25% of calories, it will start to depress your fat-metabolism and again you'll be fuelling yourself from glucose produced from the protein. It'll also raise insulin more and so lock the fat that you want to be burning during your fast.

One possible problem is that your body is simply not releasing its stored fat, for any of a number of reasons, I remember reading a study showing that in overweight individuals (not that I'm suggesting you're overweight!) they were less able to access their stored fat than lean ones (who knows which way round causation runs), perhaps it's due to higher insulin.

I'm interested to how this 4-5 hour limit applies in practise though. If you eat dinner a few hours before bed (say 3) then you'll have been fasting around 11 hours or so by the time you wake up... would you always need to eat breakfast first thing, without extending the fast? I always include sleeping time in my intermittent fasts, both because it seems like the best time for the body to focus on repair etc and also because I rarely want food just before or just after sleep.