Ok, apparently I have a 4S. Maybe it's older than I thought.

I know someone with a 4S and the battery really struggles now especially when using GPS.

This happened to me. I bought a Mophie Juicepack for it and it solved the problem...by doubling the weight/volume of the phone.

Got an iPhone 6 on friday.

Yesterday I went on a 2.5 hour, 30 mile bike ride, and the iphone6 battery meter was essentially still at 100%. My 4s would have been well under 50%. Of course, the M8 is why, but after 3 years of recharging the 4s every night the battery was really starting to feel it.