Counter-intuitively 4 nodes are less reliable than 3 nodes for majority vote systems. Both can tolerate single failed node. However probability of 2nd node fail

Counter-intuitively 4 nodes are less reliable than 3 nodes for majority vote systems. Both can tolerate single failed node. However probability of 2nd node failure is 50% higher for 4 node setup than for 3 node setup. That's because 2nd node failure probablity for 4 node system is P(a) + P(b) + P(c) = 3P and for 3 node system is P(a) + P(b) = 2P; it's 3P vs 2P probablity. It's more than 50% overall because similar probability happens for first node failure - it's more likely that one out of 4 nodes will fail than one out of 3. In other words you have to guarantee one more node running without any reliability gains and that guarantee costs reliability.