The ghost line option isn't very good on that plan unfortunately. Best bet is to sell the iPhone and that should cover the bulk of the s4 purchase.
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Current Devices: Nexus 5, HP 14 Chromebook (Data Enabled), Google Glass, (2013) LTE Nexus 7, Chromecast
Retired: Samsung Fascinate, Droid Charge, Galaxy Nexus, (2012) WiFi Nexus 7, Asus TF700, Nexus 4, Logitech Revue