You can absolutely run 2 data/master-eligible nodes plus a single master-eligible tie-breaker node as a safe configuration. The only constraint is that you should have an uneven number of master-eligibile node to avoid a split brain. You also need to understand what the resilience guarantees are for any given number of replica (roughly: each replica allows for the loss of a single random node) and how many replica you can allocate on a given cluster (at most one per data node). That would allow you to run a 2-datanode cluster in a configuration that survives the loss of one node.
I've been doing ES consulting and running clusters since 0.14 and I see very few clusters that run more than a single replica. Most 3 node clusters I see are run with three nodes because you can then have three identical configurations at the cost of throwing more hardware at the problem.
> but do you really want all your traffic to hit this one instance when the other one goes down
Whether that's a problem really really depends on whether your cluster is write-heavy or read heavy. Basically all ELK clusters are write heavy and in that case, loosing one of two nodes would also cut writes in half (due to the write amplification that replica cause). Other clusters just have replica for resilience and can survive the read load with half of the nodes available. Whether that is the case for your cluster(s) depends - that's why I explicitly asked what constraints the OP had.
I pretty much specifically avoid 4 node clusters. You’d have to either designate 3 of the four nodes as master-eligibile with a quorum of 2 or have all of them master-eligible with a quorum of 3. Both options allow for failure of a single node before the cluster becomes unavailable. Any other configuration would either fail immediately on a node loss (quorum 4) or be unsafe (quorum of 2, allows for split-brain)
I’d much rather opt for 4 data/master eligible nodes plus a dedicated master eligible node with a quorum of 3.
You also need to pick the number of replica suitably: each replica allows for the loss of a single random(!) data node while retaining all your data. Note that if losses are not random but you want to safeguard against loss of a rack or an availability zone or such, configurations are possible that distribute primary and replica suitably (“keep a full copy on either side”)