HANA is a database that you can use instead of MsSQL, Oracle or DB2 in your systems
S/4 is the incompatible successor of ECC (itself the successor of R/3), the main ERP product of SAP. It only runs on HANA, while R/3 worked with ASE, DB2, MaxDB, MsSQL, or Oracle.
New data model, for example in Financials table ACDOCA replaces COEP, COBK, ANEK, ANEP, ANEA and several other tables
Some less used modules are removed, some are added
There are other smaller differences, like matnr field becoming 40 characters instead of 18