Hi Attila,
Your first question was if Gateway will support it. As the SAP Roadmap says, it is as planned.
In terms of UI library, the SAPUI5 1.38 has the OData V4 (as experimental) but you are not able to consume OData V4 from Gateway at this moment using NW 7.5 SP3, because the Gateway still supporting V2 only.
The SAPUI5 OData V4 library can consume any third party which provides V4 as well.
In summary, the SAPUI5 is V4 (1.38) but the SAP backend still in V2 (NW 7.50 SP3).
Cheers,
Valter