You asked to allocate enough bytes for a struct _acme. As the struct _acme struct shouldn't have any padding, this should be the size of an int.

An int must be at least 16 bits in size. On a desktop system, I expect it to be 32 bits, and I expect CHAR_BIT to be 8. On such a system, you asked to allocate 4 bytes.

According to what you're saying, malloc allocated more than you requested. That's entirely reasonable, especially if the size returned includes the internal headers. It's also possible there's a minimum size the system can allocate.

Whatever the reason, you may not use any of the memory beyond what you requested.

From man7's page on malloc_usable_size():

The value returned by malloc_usable_size() may be greater than the requested size of the allocation because of various internal implementation details, none of which the programmer should rely on. This function is intended to only be used for diagnostics and statistics; writing to the excess memory without first calling realloc(3) to resize the allocation is not supported. The returned value is only valid at the time of the call.