If u go to 3,4 days trekking where u could not find any place to recharge ur Android smartphone then obviously GPS will not work.
Although I did not use GPS on Android smartphones in trekking but I want to give some suggestions.
GPS consumes much power when it is used continuously. So u will have to carry 2,3 extra portable batteries for 3,4 days trekking in order to keep ur cell phone ON. These Battries are costly e.g Samsung S3's battery not less than 1800 Rs. I also found that accuracy of smartphone's GPS are not comparable to GPS devices like Garmin GPSs.
So its better u purchase any good and cheap GPS device. Its more reliable and long life and consumes ordinary dry cells.
I suggest u to take Garmin eTrex 10. Its price is 10k.
http://www.garmin.pk/index.php/etrex-10.html