1) Do we need to have two additional dedicated Stream Nodes?
Ans:- One Stream nodeType is mandatory & As per the installation guide having 2 Stream nodes is recommended to avoid the single point of failures.
2) If we keep our current 4 node system, which nodes should we add the Stream node type?
Ans:- If possible, it is recommended to isolate Stream nodeType from WebUser nodes. Considering the above 4 node system, you may either pick any one or two nodes (which are not web user nodes).
3) What are the considerations for having multiple node types for a single node?
Ans:- Effective usage of system resources? Like instead of having a dedicated node for every nodeType.
Based on the agent's processes, If the system permits You may very associate Stream,BackgroundProcessing,Search,BIX nodetypes with a single node or 2 nodes or distribute it like below based on your usage.