As of now JBoss Redhat EAP 7.1,7.2 and 7.3 are supported. 7.4 has not been tested and certified yet.
Beginning in Pega 8.6, the EAR deployment option is being deprecated. This includes EAR deployment options for JBoss, Weblogic and Websphere. In the upcoming 8.7 release, EAR based deployment options will not be supported.
For more details refer the platform support guide.
Pega V8.7 / V8.6 on JBoss EAP 7.4 - answer: 7.4 has not been tested and certified yet.
Pega V8.7 on JBoss EAP 7.3 - answer: already supported
Your question: What is the release plan for the EAP 7.4 support in 8.6.
Answer: Most of the time the latest platform versions work without any regressions. But sometimes it breaks backward compatibility. The plan is to certify JBoss Redhat EAP 7.4 for Pega 8.8 (due out latter part of this year). This means that as long as there are no backward regressions found at the time 8.8 will be released, it will also be certified for earlier releases.