[carlmr]

他是 is tāshì which doesn't transform I think. Did you mean to write 你是 nǐshì? I think that transforms differently though. With the half 3rd tone only dropping.

The classical example is 4/4 不是. Which goes bùshì -> búshì.

Or 3/3 that becomes 2/3. E.g. 你好 nǐhǎo becoming níhǎo.

The 1/4 -> 2/4 transformation I think is specific to one. 一个 yīgè becomes yígè.