Redshift - Four ra3.4xlarge vs a single ra3.16xlarge - what is the performance advantage (if any)?

Ra3.16Xl has a advantage of larger leader node. For certain read heavy workloads, Ra3.16xl would help improve performance, but a single Ra3.16XL node is not supported. You would need a minimum of 2 nodes for Ra3.16Xl.

If your CPU is 100% and queries are not performing as expected you can look into

1. Scale out - Add more Ra3.4xl nodes (Once you have 8 nodes of Ra3.4xl, switching to 2 nodes of Ra3.16xl would be price neutral.)
2. Make sure tables have the right distribution and sort keys. https://docs.aws.amazon.com/redshift/latest/dg/c_designing-tables-best-practices.html
3. Make sure concurrency scaling is enabled to handle spiky workloads. https://aws.amazon.com/blogs/aws/new-concurrency-scaling-for-amazon-redshift-peak-performance-at-all-times/
4. Make sure WLM is configured correctly. https://aws.amazon.com/blogs/big-data/manage-your-workloads-better-using-amazon-redshift-workload-management/