By 'Part' I'm assuming you mean 'Sequence'. In Logic, a sequence is a coloured block on the arrange window, containing midi data.

The answer to your problem should be very simple. If you look at the transport bar (the one with the play, stop etc. buttons on it) you'll see a few black data displays on the right hand side. Depending on your setup there should be two rows of them, and the default position for the timing information window is the top row, second from right. This shows the timing signature ('4/4' usually) and the 'snap to' setting underneath. In your case it should be '/4'. Hold your mouse over this, press and hold your mouse button, and move the mouse up or down.

Hope this is what you're looking for!

Tequila