I am hypercritical about people's public speaking and speech-making abilities so I may be of help!
For this sort of speech, always always always come from a place of sincerity and realness. People will see straight through your speech if you seem overly concerned with being "inspirational". Also, if you start giving advice for the sake of giving advice, you’ll just come across as preachy – especially since sports are riddled with platitudes.
To brainstorm for this speech, start by thinking about the specifics of your 4-year experience. Do not start out by looking at trite sport sayings or cliché quotes and then spinning your own personal experience to fulfill or prove them.
Did you ever have a major epiphany or transformation in perspective that was spurred on by being on this team? This could come from a momentous occasion when emotions were heightened (e.g. a great victory or a great failure) or from your day-to-day experience of the team (e.g. the steady pursuit of the goal, the commitment to excellence everyday, learning to take responsibility for your success).
Think about the experiences that were challenging or surprisingly gratifying. How did you react and what did you learn?
The conclusive epiphany you decide to share may be a cliché, but it will escape that cliché vibe if it came about from an anecdote or experience that is honest and specific to your time on the team. Do not force it to be something it isn’t.
I loved when seniors talked about what they were like or what the team was like when they were just freshman. This is effective for several reasons. At this point, everyone on the team is younger than you so this will immediately pique your audience's interest because everyone's curious about what you or the team was like "way back when". You're revealing a perspective that your audience has not yet heard. Additionally, this route allows for a natural flow and direction to your speech (provided the team or you were different 4 years ago). Your speech can then follow the process of that transformation that took place.
The aforementioned suggestions and prompts may not all apply to you. Remember be true to the narrative of YOUR experience. Don't say something that you THINK people will want to hear or that you THINK will bring tears to their eyes. Avoid coming across as contrived. Be specific before being broad. As cliché as it sounds, speak from the heart.