BW/4 HANA will now define SAP's future releases and hence its recommended that businesses gradually make a move from BW on HANA or BW to BW/4 HANA. BEx queries will no longer be active, reports will be based on BO tools such as SAP BO Design Studio, SAP Analysis for Office(BOA). Old infoproviers such as transient providers, Infocubes, Old- DSO's are no longer supported. The number of Data Sources reduced from 7 to just 3(ODP(for SAP source systems such as SRM), HANA source(for non-SAP) and Flat files).
All said and done, I however feel SAP was too quick to introduce BW/4 HANA as many were still upgrading to SAP BW on HANA or systems were being designed using native SAP HANA. Also, considering SAP ripping off support to SAP BW 7.3 in 2020, it is a bit too much of a push to first migrate to BW on HANA and then onto BW/4 HANA. BW/4 HANA is also not a successor of BW on HANA in that many of the objects are not supported(transient providers, for example). It is logically in series of the SAP product launches but not quite the upgrade we can think of.
It requires the system to be on BW on HANA 7.5 SP04(The transfer tool used for migration is available from SP02 itself), bare minimum pre-requisite. Although I do not doubt the efficiency and crispness of this system, when there is a migration as this, where most of the objects are not supported, companies implementing SAP systems using an older technology such as BW on HANA might really consider recharting their approaches. It is worth noting the consumers who are still on SAP BW 7.3 version and planning to upgrade gradually, the road appears to be long for them.