chore: bump to version 5.1.0-alpha.4

apps/emqx/include/emqx_release.hrl

@@ -32,10 +32,10 @@
 %% `apps/emqx/src/bpapi/README.md'
 
 %% Opensource edition
--define(EMQX_RELEASE_CE, "5.1.0-alpha.3").
+-define(EMQX_RELEASE_CE, "5.1.0-alpha.4").
 
 %% Enterprise edition
--define(EMQX_RELEASE_EE, "5.1.0-alpha.3").
+-define(EMQX_RELEASE_EE, "5.1.0-alpha.4").
 
 %% The HTTP API version
 -define(EMQX_API_VERSION, "5.0").