Even 8 hrs a day hiking with gps will put you right at the limit of battery life. Garmin says 32 hrs of gps for fenix 5x plus,, so I think 30 hrs hiking over 4 days is reasonable. As long as its being stopped but not saved each night. Other option is to use ultratrac mode. That'll definitely last the whole time.
I pretty much wear the watch every day and for me the solar/battery life have really made it so much nicer for me to use. I've gone on a week long trip where I am recording my track each day for anywhere between 6-10 hours and I ended the trip WITH battery life. (Watch was in the sun). With the Fenix 5x, I always felt like I needed to recharge.
fenix 6 / 7 series; Forerunner 45 / 45S / 55 / 245 / 245 Music / 255 / 745 / 945 (LTE) / 955 Pretty much all of the best Garmin smartwatches have Garmin Body Battery, in other words.

The Forerunner 935 is much smaller and lighter than the Fenix 5x Plus, but our test results showed it to be just as accurate. The Forerunner 935 does have fewer features and less activity tracking battery life than the Fenix, but it also has a significantly lower price tag, a much lighter weight, a smaller size, and a longer normal use battery life.

The Body Battery feature on your Garmin watch is designed to help you manage your personal energy resources around the clock. t5iDy5. 456 871 784 562 186 814 445 985

does fenix 5 have body battery