There's several ways to do that, but ultimately it might be impossible - if your song (I assume you are talking about an imported audio file that is an orange region) either doesn't have a constant beat (like it was a live performance by a human being) or if the beats-per-minute number is something like 120.25 (GB only offers whole integer values).
That being said, I find the easiest way the following:
• Cut out a piece of your song that is exactly 4 (or 8 or any other number of) bars long using the cmd-T command. As you said, you might be able to do that visually by looking at the peaks in the waveform. Don't worry about truncating the audio - it's still there and will be recovered later.
• Push the segment fully to the left. If the region is purple, make sure the 'follow tempo and pitch' box is unchecked. Adjust the project tempo with the slider so that the region fits exactly into the number of bars that you counted. (Use the Shift key for incremental steps of 1 bpm.)
• Now you can pull out your song to the left and right again and align it with the grid. Check if you really found the right tempo by recording on a dummy track (so you hear the metronome and can detect any mismatches). You might have to adjust the tempo by 1 or 2 steps in either direction.