By Akash Sriram Dec 29 (Reuters) - SoftBank Group will acquire digital infrastructure investor DigitalBridge Group in a deal valued at $4 billion, the companies said on Monday, ​as the Japanese investment firm looks to deepen its AI-related portfolio. The acquisition would ‌expand SoftBank's exposure to digital infrastructure as the Japanese conglomerate is positioning its portfolio to focus on artificial ‌intelligence. DigitalBridge shares rose about 9.7$15.27 on Monday, following a 45% rise earl ...