hi, how can I get a resource property (ip4 for example) as a string (without exports)

Not sure I understand the question - could you share a small code snippet of what you are trying to do?

console.log(computeInstance.ipv4Address) does not return a string

Ahh - you can do

computeInstance.ipv4Address.apply(console.log)

. See also https://www.pulumi.com/docs/intro/concepts/programming-model/#outputs.