Last summer we eventually had to decide to postpone the 4th CAA-GR conference due to the Covid-19 crisis. Even though the situation has not really improved since then in view of the running vaccination program and other measures there is realistic perspective that until Autumn this year it will be possible again to organize the conference including physical presence of participants. For this, we decided to give it another try and would like to invite you for the 21th/22nd October 2021. New Abstracts should be submitted until 11th June 2021. The abstracts and contributions already submitted last year will remain to be considered for presentation. There is no need for re-submission. The authors are free to update or edit their abstracts until the new deadline or to withdraw them.