Load balancer that works at the transport layer, routing traffic based only on IP address and port number without looking at the content. Like a mail sorter who only reads the address on the envelope, not what's inside.
A Layer 4 load balancer routes all traffic on port 443 to backend servers based on IP, handling millions of connections per second with very low latency.
All four provide Layer 4 (TCP/UDP) load balancing that routes based on IP/port and connection state, prioritizing high throughput and low latency. Feature sets differ (e.g., TLS termination, logging, global vs regional options), but the core L4 behavior is comparable.