This poem has the classical pattern in rhyme: A, B, A, B... in the 3 quartets, until the couplet which rhymes A, A. It’s a End Rhyme, the most common amongst the rhytmic patterns, and which occurs at the end of two or more lines. It is a true rhyme; the sounds are nearly identical:
B - ... my teeming brain,
A
B - ... full-ripen'd grain;
A - ... and think,
A - ... do sink.
Except in the 1st and 3rd lines where we have a weak rhyme, also called slant, oblique, approximate, or half rhyme, that refers to words with similar but not identical sounds:
A - ... cease to be
B
A - ... in charact'ry