Bodhicitta and the Arhat
Bodhicitta is 1. the intention to liberate all beings and attain buddhahood; 2. the true nature of mind, compassionate emptiness"when one sees that form if Voidness, he accomplishes the great Wisdom, and he abides no more in samsara. When one sees that the Voidness is form, he attains the great compassion and will no more remain in Nirvana. Because form and Voidness, Wisdom and compassion, have all become non-differentiated, he is able to practice the non-abiding acts."
(An Excerpt from the Commentary on the Heart Sutra by Master Fa Tsang, The Buddhist Teaching of Totality: The Philosophy of Hwa Yen Buddhism by Garma C. C. Chang, p. 204.)"There is no emptiness meditation not permeated by compassion;
For the practice of compassion is solely [the practice of] emptiness.
As for emptiness, even those seeking tranquil abiding must practice it,
For this vehicle, however, emptiness is compassion;
And the self-nature of this compassion is emptiness.
So understand that compassion is the essential nature."
(Atisa: Advice to Namdak Tsuknor, Mind Training: The Great Collection by Thupten Jinpa, p. 267.)"The practice of "clear observation" will cure the followers of the Hinayana of the fault of having narrow and inferior minds which bring forth no great compassion, and will free ordinary men from their failure to cultivate the capacity for goodness. For these reasons, both "cessation" and "clear observation" are complementary and inseparable. If the two are not practiced together, then one cannot enter the path to enlightenment."
(The Awakening of Faith: Attributed to Asvaghosa, tr. by Y. S. Hakeda, p. 95)
The practice of "clear observation", i.e. vipasyana is described as contemplating suffering and impermanence. Indeed, the common four bodhisattva vows are very much the other side of the four noble truths. In Mahayana it is understood that emptiness and compassion go hand in hand, in fact, they're inseparably and ultimately the same. Consequently, if an arhat realises the four noble truths (of which the third is claimed to be the final meaning and equal to buddha-nature in the Srimaladevisutra) he also accomplishes great compassion, fulfilling the requirements of ultimate bodhicitta.
Something interesting to read: Arahants, Bodhisattvas, and Buddhas
by Ven. Bhikkhu Bodhi