Firstly, having 4 separate 2 node clusters with one FCI deployment in each seems to me to be a little extreme. The second model you describe does not really make sense to me. Instead of either deployment I would suggest running 2 Clusters of 4 nodes (you would need a witness for each) and consider one node in each as a dedicated failover node (i.e. that you install 3 SQL FCIs to each cluster).
Also you should try to deploy on Windows 2012 R2 because of its dynamic quorum and dynamic witness capabilities (2012 only had dynamic quorum) -which will help to maintain maximum uptime upon node failure.
As you will be aware, the biggest issue with not having enough dedicated failover nodes is that ultimately you are exposed to SQL co-existence considerations (such as setting the correct max-memory upon failover if shared with other SQL FCIs amongst other things). Furthermore in truly Highly Available environments you probably would not want to fail-back post failover (otherwise it is a further addition of down-time), so sharing dedicated failover nodes would probably necessitate failback of one instance (assuming two had failed over).
However, that said, all system deployments are a compromise and provisioning too many redundant failover nodes does not make sense and it is important to find the right balance between HA requirement and cost effectiveness.
As a general rule of thumb you should consider an approximate 1 dedicated (failover node) per (2 SQL FCI) 3 node cluster, 2 dedicated per (5 FCI) 7 node cluster and 3 dedicated per (8 FCI) 11 node cluster.
These figures are generous approximations from nearly 20 years personal experience of SQL Failover Clustering but obviously there are many other factors that can affect your design strategy.
p.s. There are some technical Clustering reasons why it makes sense not to deploy a huge number of cluster nodes so I would probably not go beyond a 5 node cluster anyway (and in that case I would probably still be happy with 1 dedicated failover node).
