How do I choose a light GPS device supporting Strava? Up til now I have ridden with Strava running on my mobile phone sitting in a special mobile phone holder which sits up on my handlebars.

Now I am looking for a better solution, both more aerodynamic for my new bike, and also with longer battery life (My present phone only lasts 2 hours with gps running).

How do I select the lightest / best device that runs Strava? It obviously needs to be GPS capable.

What features do I look for? Is it a smart phone? A smart watch? A garmin device?

The device being light, comfortable, reliable and able to record rides of 4 hours or more would be ideal.

Thanks for any suggestions.
The 'best' is fairly subjective and probably not compatible with lightest. For what it's worth I remember reading somewhere that the Edge 510 is widely used through the pro peloton.

I use a Garmin Edge 810 and would recommend it in an instant but it is possibly bigger than it needs to be if you're very worried about weight and size (I chose it for its navigation and live tracking).

On the smaller scale, the Garmin Edge 200 or slightly larger 510 might work.

Also the Cateye Stealth 10 (or 50 if you want Cadence) are supposed to be highly rated. 

I'd be surprised if any of them struggled with lasting far longer than 4 hours. My 810 handles 5 hour rides with about 50% battery left and that's using bluetooth for live tracking for the whole ride.

The downside of a pocket type GPS tracker is that you don't get the data (like speed, cadence) in front of you while riding.