All named files are compiled to object code modules, but are not linked. You can use the -o option to name the output if you just supply one input filename.
The following command compiles file1.c and file4.c to produce the object files file1.o and file4.o.
cc -c file1.c file4.c
The following command compiles file1.c and produces the object file obj/file1.o.
cc -c file.c -o obj/file1.o