I said, "As long as I am healthy, I will perform four Rak‘ahs (units of prayer) in pairs after the Maghrib prayer," but I did not state it to be a vow. However, sometimes, I feel really exhausted, and I might be outdoors or in a hurry, and cannot perform them. What should I do? Should I perform the four Rak‘ahs instantly after the Maghrib prayer or whenever that is possible? Should I commit myself to performing those prayers?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

Saying, 'I will perform such-and-such Rak‘ahs after the Maghrib prayer', is not considered a binding vow that should be fulfilled, according to our view, unless you held the intention of taking a vow when you uttered this statement. A vow is valid and binding when the vow-taker utters a vow-formula suggesting commitment, such as, 'I vow to Allaah, the Exalted, to do such-and- such', and the like.

The formula that you used does not suggest commitment, it is more like a promise. From an Islamic point of view though, it is desirable to deliver on such a promise, although it is not an obligation. If a person  intentionally and willingly fails to fulfill it, then he bears no sin for that.

Allaah Knows best.