In V4 we use the Header Content-Transfer-Encoding:binary together with the Content-Type: application/http in batch payloads (exactly as in V3 ) and in async responses.

In HTTP 1.1 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.1 and https://tools.ietf.org/html/rfc7230#section-8.3.2) the media type application/http is defined "...Encoding considerations: HTTP messages enclosed by this type are in "binary" format; use of an appropriate Content-Transfer-Encoding is required when transmitted via E-mail."

And in a section about differences between HTTP and MIME entities, the spec states (http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.4.5 and https://tools.ietf.org/html/rfc7231#appendix-A.5): "HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 2045. [...]"