Although WiMAX allowed Sprint to be the first operator in the US with 4G, WiMAX was quickly eclipsed by LTE which became the predominant 4G technology worldwide. Sprint announced in 2011 that it would abandon WiMAX in favor of LTE. Sprint's last WiMAX device was the Samsung Galaxy S II, which Sprint called the Samsung Epic 4G Touch, released in Sept 2011.
Sprint stopped selling WIMAX devices to postpaid customers at the end of 2012, so there probably aren't a lot of WiMAX devices still active on postpaid accounts. All of Sprint's WiMAX phones and most of its mobile hotspots support EVDO 3G as well as WiMAX and will continue working as 3G only devices after the WiMAX shutdown. Sprint has said that it will offer WiMAX postpaid customers a free or low-cost upgrade to a LTE device.
Sprint has continued to offer dual-mode 3G/WIMAX devices on the Boost and Virgin Mobile prepaid brands throughout 2013 and 2014. Even today, you can buy a 3G/WiMAX hotspot from Virgin Mobile for $71.99. Sprint has said it will notify its prepaid WiMAX customers 180 days before the shutdown, which would be May 10. 2015. I doubt that prepaid WiMAX customers will get free phones or hotspots because their current ones will continue to work as 3G only devices, but hopefully they will at least get a discount on a LTE device.
Sprint MVNO FreedomPop has sold a lot of WiMAX only hotspots and USB sticks. These will stop working 13 months from now. FreedomPop has a device swap program that lets users trade their WiMAX only device for an LTE one by paying the difference in price. FreedomPop's only current LTE mobile broadband device is a hotspot priced at $149.99 plus $9.99 shipping. Most of FreedomPop's WiMAX-only devices were sold for less than $50 so it will cost most customers $100 or more to swap currently. Hopefully LTE device prices will go down between now and the WIMAX shut down late next year.
Sources: Wireless Week, FreedomPop Forums. Image: WiMAX Forum