I am old. I remember when sizeof(int) was usually 2. And I may live long enough to see sizeof(int) typically be 8.
The 16-to-32 bit transition broke a lot of code that assumed sizeof(int) was 2. The next transition may do the same. (Or, we may keep "int is 32 bits" forever, and use long for 64. Who knows? I don't. You probably don't, either, so don't assume that sizeof(int) = 4.)
The present behavior on normal 32-bit and 64-bit platforms is that, on both, 'int' is 4 bytes, and 'long long' is 8 bytes; and on 64-bit, whether 'long' is 4 or 8 bytes depends on the ABI/target OS.
I'd imagine it's quite likely that 'int' stays 4 bytes even on hypothetical 128-bit CPUs - there's not much reason to change it, as on 64-bit it's already less than the CPU width, and thus is pretty much arbitrary even today. But yes, anything that wants a 4-byte/32-bit integer should just use <stdint.h>'s int32_t.
The 16-to-32 bit transition broke a lot of code that assumed sizeof(int) was 2. The next transition may do the same. (Or, we may keep "int is 32 bits" forever, and use long for 64. Who knows? I don't. You probably don't, either, so don't assume that sizeof(int) = 4.)