"Po čtyřech letech zase začal pít."

Translation: After four years, he started drinking again.

[dsarkarati]

Po čtyřech letech zase začal pít.
"He started to drink again after four years." is identical in meaning to "After four years, he started drinking again."

[LukasBlech]

"After four years, he started to drink again." is not possible?

[VladaFu]

It is possible and accepted.

Unfortunately we have no report from you so we have no idea what might be the issue, if you made a typo or if it is a Duolingo bug...

[Steven2765]

Why not: "after four years he began to drink again"?

[BoneheadBass]

"After four years, he began to drink again" is also accepted.

[Jeff448012]

Is there a solid rule about the usage of "let" vs "rok"? I thought it was the rule of 5 or more, i.e., čtyři roky, pět let. But that doesn't seem to be the case.

[VladaFu]

No, there isn't, it wil depend on the case, context... Better observe the actual usage. "pět roků" is completely correct as well.

[Stephan881810]

what is wrong with " after four years he again started drinking"

[sadalicious1]

It sounds quite unnatural in to me (native AmE). Adverbs generally go after the verb.