Hi Christoforos,
why don't You ask Your other question here at the forum:
is the componentn available for jooml 4 event in a beta stage ?
I don't know which Joomla 4 event You mean, but my information for Joomla 4 is, that the core team developers promise, all extensions, which work with Joomla 3.9 also will work with Joomla 4. The current version of BwPostman works with Joomla 3.9, so there is no need for a beta for Joomla 4.
By the way, the mail address You entered at the contact form does not work:
<This email address is being protected from spambots. You need JavaScript enabled to view it.>: host aspmx.l.google.com[74.125.140.27] said:
550-5.1.1 The email account that you tried to reach does not exist.
Hth
Romana