The simple answer is that a 4G network theoretically will have a higher data transfer rate. With the appropriate amount of spectrum and good network engineering, a Long Term Evolution (LTE)-based network has the potential to reach 100 Mbps, while a WiMAX network can top out at 70 Mbps.
A more complex answer is that a 4G wireless network is a pure data connection: that is, it is an end-to-end Internet Protocol connection. This provides some real advantages, but also some disadvantages. On the one hand, a smartphone simply becomes another data device whose native mode is as an Internet-enabled terminal and that can be managed as such.
On the other hand, services such as voice require some additional machinations to support effectively. Since voice is not intrinsically data-centric and must be converted to data before it can be transferred, voice-capable LTE handsets have been slow in coming.