The first Pyara who got up to offer his head to Guru Sahib was Bhai Daya Singh signifying the fact that the start of Dharma (religion) is with Daya (compassion).
Daya or Compassion holds a very important place in Gurmat Dharma; so much so that it is considered to be a prelude to Dharma. For centuries some people held the belief that this Earth is held by a Bull on his horns but Siri Guru jee in Siri Jap jee Sahib made it clear that the Bull that is holding the Earth is Dharma:
ਧੌਲੁ ਧਰਮੁ ਦਇਆ ਕਾ ਪੂਤੁ ॥ ਸੰਤੋਖੁ ਥਾਪਿ ਰਖਿਆ ਜਿਨਿ ਸੂਤਿ ॥
Guru Sahib is making it clear in the above Gurvaak that the Bull holding the Earth is Dharma and this Dharma in return is a product of Daya or compassion. Where there is no Daya, there can be no Dharma and where there is Dharma, there ought to be Daya. Where both Daya and Dharma are present, Santokh (Contentedness) too comes to reside. In other words a Dharmi person is always compassionate and contented.
Next Pyara after Bhai Daya Singh jee was Bhai Dharam Singh, in a way, reinforcing the fact that where there is Daya, there is Dharma too. A Sikh is Dharmi only if he has Daya.
The next Pyara who offered his head to Guru Sahib was Bhai Himmat Singh, teaching us that where Daya and Dharma are present, Himmat (courage, valour) can’t be absent from there.
The fourth Pyara was Bhai Mohkam Singh, signifiying that when Daya, Dharma and Himmat all mingle, they become Mohkam (a Farsi word meaning very strong, determined).
The last Pyara was Bhai Sahib Singh and his name and inclusion symbolizes that fact that where there are Daya, Dharma, Himmat and Mohkam, there is Sahib (meaning Master i.e. Vaheguru).