Layer 4 - Process Layer or Application Layer - This is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
Layer 3 - Host-To-Host (Transport) Layer - This is where flow-control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring that packets are in fact received.
Layer 2 - Internet or Internetworking Layer - This layer defines IP addresses, with many routing schemes for navigating packets from one IP address to another.
Layer 1 - Network Access Layer - This layer describes the physical equipment necessary for communications, such as twisted pair cables, the signalling used on that equipment, and the low-level protocols using that signalling.