I am replacing the rear leaf spring and do I need a 4 wheel alignment afterwards?
No, you should be fine. 4 wheel alignment is typically done on all-wheel drive or 4wd vehicles so the tires aren't "fighting" each other. You should be fine just replacing the leaf springs.
I have to disagree re:littlehorn's post. Anytime you change a suspension component, you usually change your camber/caster adjustment of the rear end as a result. I would guarantee the toe-in would need to change since the stance will be higher. Get the $60 alignment to save $400 for new tires. BTW, alignments should be done everytime new tires are bought, too, regardless of what kind of car you drive.
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