Add support for n2-standard-4 machine type.

[cpgaffney1]

Description

Add n2-standard-4 as a supported machine type in system_characteristics.py.

Issue

N/A

Testing

Tested by demonstrating successful cluster creation for the desired machine type:

xpk cluster create --cluster=cpgaffney-n2-standard-4-64-32 --device-type=n2-standard-4-64 --num-slices=32 --custom-cluster-arguments= --network=cpgaffney-privatenetwork  --subnetwork=cpgaffney-privatesubnet  --scopes=storage-full,gke-default  --enable-ip-alias  --master-ipv4-cidr 172.16.0.32/28  --cluster-ipv4-cidr=10.224.0.0/12  --enable-private-nodes  --no-enable-master-authorized-networks  --custom-nodepool-arguments= --scopes=storage-full,gke-default  --enable-gvnic  --max-pods-per-node 15  --disk-type=pd-standard  --disk-size=50  --spot

[jamOne-]

thanks