Data in Hazelcast is both active data and backup data for high availability, so the total memory footprint is the size of active data plus the size of backup data. If you use a single backup, it means the total memory footprint is two times the active data (active data + backup data). If you use, for example, two backups, then the total memory footprint is three times the active data (active data + backup data + backup data).
If you use only heap memory, each Hazelcast member with a 4 GB heap should accommodate a maximum of 3.5 GB of total data (active and backup). If you use the High-Density Memory Store, up to 75% of the configured physical memory footprint may be used for active and backup data, with headroom of 25% for normal memory fragmentation. In both cases, however, you should also keep some memory headroom available to handle any member failure or explicit member shutdown. When a member leaves the cluster, the data previously owned by the newly offline member will be distributed among the remaining members. For this reason, we recommend that you plan to use only 60% of available memory, with 40% headroom to handle member failure or shutdown.