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Terminal 4 and 5 or close and it is easily walkable. There are airside connections, but there is a lot of construction happening and it's hard to determine what the current state is. There is an airside tunnel between T4 and T5
Here are some good resources: https://onemileatatime.com/connect-terminals-lax/, https://milestomemories.boardingarea.com/lax-airside-connections-between-terminals/
You should get both boarding passes when you check in for your first flight. In LAX you just have to get off the arriving plane, look up the gate for your departing flight and follow the signage to this gate. 3 hours is plenty for that unless there is a major delay with your first leg.