What is the best external battery charger for iphone

I see the battery of this site is good, maybe it will suitable for you

http://www.external-battery.net/productstype/external-battery-1000-3000mah.htm

do you want an external battery - or a battery case ( a case with a battery built in) both cases searching amazon will find you lots of options - for battery cases - i have a Mophie ( they have a few models for the 4S - a company called Third Rail Mobility also makes a battery case for the 4S

http://www.thirdrailmobility.com

Ivyskin is another option for a battery case (with a builtin screen protector)

htp://www.ivyskin.com

and as I said looking on amazon will give you plenty of options for either an external battery or a battery case