How come t-mobile iPhone 4s has 4g and my sprint 4s does not have 4g?

How come t-mobile iPhone 4s has 4g and my sprint 4s does not have 4g?

Maybe Sprint is not as good as T-Mobile.

In the United States, T-Mobile and AT&T iPhones that are connected to an HSPA network will display 4G. HSPA is technically a 3G technology, however HSPA+ can offer speeds similar to today's LTE and WiMAX networks. (Up to a theoretical 42Mbps.) In 2010, the ITU-R (the organization that sets the standards for 4G technologies) made a ruling that HSPA+, LTE, and WiMAX could be labeled as 4G (Pre-4G), even though they don't meet the true-4G standards.

The T-Mobile iPhone 4S runs on this HSPA+ network, and so it demonstrates a 4G symbol. On Sprint, EVDO (CDMA flavour of 3G) has a maximum theoretical speed of 3.1Mbps. Due to this reason Sprint and Verizon's EVDO networks can't reach speeds similar to 4G, and are unable to label their service as 4G.

The iPhone 4S doesn't contain any actual 4G radios (LTE or WiMAX), so it is unable to connect to any actual 4G network.

2 different carriers, therefore 2 different 4G coverage areas. You may simply be in an area where T-Mobile has it but Sprint does not.