So I bought a used 4S and I never expected the battery to be great, but it's acting really weird. It shuts itself down sometimes when it's under 41%, mostly while trying to open snapchat or something smilier, but also while watching Netflix. Then it says it's out of battery, but when I plug it in and it turns on, it has the same percentage as when it shut down. Also, at one point it had 25%, then I plugged the AC in and it said 31% right away. After I unplugged it again, it went down to 21%. My main question is, does this sound like the battery would be replaced, a software issue or something else entirely?