L’Baiet 8 Inch Gel Memory Foam Mattress, Twin Size - Bed in a Box - Medium Firm - Breathable Cooling-Gel Comfort Technology - Pressure Relief Comfy Support Soft Fabric Cover - CertiPUR-US Certified

4" is a topper - return it. Minimum mattress size is typically 8" - no specifics on quality but example here - you should be able to find something for $100-$200 depending on bed size. Generally you'd want 10" but if you don't have a choice you don't have a choice. At this point you do not have a mattress. The only alternative to this is tossing it on the floor and using it there but it will definitely be rough on your back either way, just better than the slats you're likely laying on with sections completely unsupported. If using it on the floor, flip it often and allow it to air out frequently to reduce risk of mold.