Buying a 4G enabled device is not the problem, but knowing the bandwidth of devices compatible with the sim you wish to use.
I would list out the bandwidth of Mtn Ntel Glo and Etisalat bandwidth.
4G LTE Bands OF MTN, GLO, ETISALAT AND NTEL
1. Ntel 4G supports LTE band 3(1800) and band 8(900)
2. MTN 4G supports bands (800/1800/2100/2600MHz)
3. Glo 4G supports band 28(700MHz)
4. Etisalat 4G supports band 3(1800MH)
I will advise you to buy a smartphone with 4G LTE Bands 3(1800), band 8(900), Band 20(800), Band 28(700). Though, getting a phone with all the bands is not easy. Anyway, we expecting Airtel to roll out their own 4G LTE network.