1. ddb is the kernel debugger. Your kernel entered a panic. More information should be right -above- the ddb> prompt as for the reason for the panic. See this man page for general guidance on panics and crashes:
http://www.openbsd.org/cgi-bin/man.c...86&format=html
2. 4.7 is not -yet- supported. It will become supported the day that 4.7 is officially released by the project. Right now, only 4.5, 4.6, and -current are supported.