Dear Colleagues, The 4th RD51 Collaboration meeting will take place at CERN from November 23rd to November 25th, 2009. The program will consist of plenary sessions and parallel working group (WG) meetings on techniques, detector physics issues, simulations, applications, electronics, and common production and test facilities. The opening plenary session will take place on Monday, November 23rd in the morning. Based on your feedback and experience from the previous meetings there will be only two WG parallel sessions scheduled at the same time on Monday afternoon and on Tuesday. On Wednesday, November 25th there will be closing plenary sessions with the reports from the working group conveners, short presentations of industrial companies and Collaboration Board news. We do not intend to ask for registration fees. The Collaboration dinner will be held on Tuesday, November 24th (to be paid at the registration in cash). If you are planning to attend the meeting and present your work please contact the corresponding working group convener and to register. We are looking forward to see you at CERN ! Best regards, Leszek and Maxim.