Q:Does Rapid.Space allow to access the "outside internet" over IPv4?
A: Yes, with some limitations. Outgoing IPv4 traffic must support NAT and is limited to a whitelist of destination hosts. For most applications, it is enough.
For applications that need to remove all limitations, one possibility which costs a few Euros per month is to connect Rapid.Space VM to a third party provider (ex. Online or Kimsufi) through an IPv6 tunnel and route all IPv4 traffic through this tunnel using 4in6 protocol for example.