Question (Mishnah in Perek 4): 'One who separates Terumah by counting is praiseworthy, by measuring is more praiseworthy and by weighing is the most praiseworthy'. But in our Mishnah all of these were prohibited?

Answer (R. Yehoshua ben Levi): Our Mishnah refers to Terumah Gedolah; that Mishnah refers to Terumas Maaser (which must be an exact amount).

Support (Baraisa) (Eliezer ben Gomel): From where is it known that one may not separate Terumah according to measure, weight or number? As the pasuk states (Bamidbar 18:27), "And your Terumah shall be considered for you'' - separate Terumah through consideration (i.e. estimation) and not through measure, weight and number. Just as Terumah Gedolah is through estimation, so is Terumas Maaser.

Question: Didn't the Mishnah teach that one separates (Terumas Maaser) through measure, weight or number?

Answer (R. Elazar): Explain it like this - A person can measure his Tevel and bring it into his house, as long as he doesn't separate its Terumah through measuring. A person can weigh his Tevel and bring it into his house, as long as he doesn't weigh out his Terumah. A person can count his Tevel and bring it into his house, as long as he doesn't count out his Terumah.