Is it possible to attach ~30 persistent disks to single k8s node (e.g. n1-standard-4)?
According to the documentation 2-4 core node can support up to 64 attached disks in Beta: Link.
Is it supported by GKE? Is there any limit in GKE Kubernetes?
GKE has the same limitation as vanilla Kubernetes on GCP per se. The Kubernetes limits for the largest public cloud providers are documented here
You can also change those limits using the KUBE_MAX_PD_VOLS on the kube-scheduler (After restarting). Unfortunately, you won't be able to change this on GKE yet, because GKE doesn't give you access to the master(s) configuration yet.
Also documented here is Dynamic Volume Limits introduced in Kubernetes 1.11 and currently in Beta.
I believe you self-answered your first question, the n1-standard-4 VM has 4 vCPUs and per the link that you provided you can attach up to 64 disks. So yes, you should be able to attach 30 persistent disks, a PVC/PV in the GCE storage class maps to GCP VM disk.