When you say that "BGP community attribute is limited to 4bytes in total by the standard", which standard are you referring to? Mikrotik's standard? IANA's standard? It is quite unclear.
Support for 4-byte AS numbers should be considered a priority, as there are a fair few carriers, transit providers and IXes who require you to peer with their 4-byte ASN. Should you wish to use their communities, you'd be in trouble.
From my understanding, if an upstream peer uses (for example) 65540:666 to blackhole routes up to a /32, anyone who uses a Mikrotik router for peering with them cannot blackhole route the prefix using communities. They would have to create a static route, which could get messy.
I intend to use BGP communities to be able to prevent upstream carriers from sending my routes to certain transit and IX providers, as well as allowing me to offer my clients the same featureset. I want to be able to allow my customers the ability to not have their routes filtered through to certain transit and IX providers that I peer with. And no, it is not possible to use a value that fits, as it is common practice to use BGP communities in the format 65536:nnn, as my AS number is 4-bytes (6 digits). IANA specifies which registries are assigned which AS numbers (see https://www.iana.org/assignments/as-num ... bers.xhtml
) , and there is no allowance for scenarios like this.