As soon as task #1 completes, tasks #2, #3, and #4 can start. Task #2 is expected to take 5 days, the longest of the three, so it is on the critical path. Task #3 is expected to take 4 days so it has float of one day, the difference between the time it is expected to take (4 days) and the time available to complete it (the 5 days scheduled for task #2 which is being worked in parallel). Similarly, Task #4 is expected to take 2 days so it has a float of three days (5 days for task #2 minus the 2 days for task #4).
If all the float on a task is used up, that task may become a task on the critical path.