The 4s was the first dual-modem (CDMA/GSM) iphone. The GSM side, if locked by Sprint, can certainly be unlocked by them, but not using any code (there is nowhere to enter any unlock code). You unlock an iPhone with iTunes, once the carrier has informed Apple to provision it for release of SIM lock - see https://support.apple.com/en-us/HT201328. You could check if it is unlocked now by trying any GSM SIM in it before you send it.
the iPhone 4s supports the 900 band used by Cubacel so as long as Sprint doesn’t mess their end up and you can unlock it in iTunes (if you don’t get the message in iTunes at the end of the restore that it was unlocked, then it is not unlocked), it should be able to connect.
You can check device compatibility by country and network, including Cuba, at www.frequencycheck.com.