The European Commission had published drafts of Horizon Europe Work Programmes for the period 2021 -2022, which include forthcoming calls for proposals. Drafts of Work Programmes are not final versions and until their official publication and adoption, may be subject to modifications. Drafts serve as indicative and informative documents for future applicants.
Although you can find in Horizon Europe Work Programme for Cluster 4 the dates of opening of the calls starting from April 2021 or beginning of May 2021, as of now (middle of May 2021) the calls of Cluster 4 didn´t open. The final versions of Horizon Europe Work Programmes and the opening of calls for proposals can be expected soon. Calls for proposals will be published than on Funding & Tenders Portal.
You can find draft of the Horizon Europe Cluster 4 Work Programme for 2021-2022 on Register of Commission Expert Groups