4週5日目 せざるをえない
Perspective・object・obligation
接続：V辞書＋しかない
there is no other way
注意 / Note：ざるを得ない is forced due to circumstances; ないわけにはいかない due to responsibility; しかない means there is no other option.
比較 / Compare：Same article: 〜ざるを得ない／〜ないわけにはいかない. These models are often mistakenly chosen for each other — carefully read the 接続 and nuances above and then decide.
よく出る形 / Common form：動辞書形＋しかない。The simplest and most neutral of the three models in the same article. Can also be used after nouns to mean "only".
電車がないので、歩くしかない。
There is no train so the only way is to walk.
失敗したら、もう一度やるしかない。
If you fail, the only way is to try again.
ここまで来たら、最後までやり通すしかない。
Having come this far, there's only one way to do it right.