汉译英句子 如果可以4年后我会回来娶你.

的确是虚拟语气，第一人称用shall比较好，after four years，4 years later常用于表示过去的将来，在这里不合适
I shall come back to marry you in 4 years, if possible.

这里主要涉及到的是虚拟语气,正确翻译应该是：
I would marry you four years later if I could.

本人觉得，你想表达一种感情，而不是生硬的翻译一句话，觉得这样可以吗？
I'll live with you ,with all my life ,if I come back four years later.Wait for me……

If I can , I'll marry you in four years time.

If you are willing to marry you, I will come back after four years

If it is possible ,I will come back and marry you four year later.