Can someone explain how segments work? I want to be able to create a malloc() function, where 4 segments will be allocated as the heap. What I want to know is, for COM files, what are the default values for the code segment and data segment? And are the remaining segments uncommitted for the programmer's use?

When COM program is loaded - the default values for all segments decided by DOS or whatever is emulating DOS (DOSBOX, NTVDM, etc).

If you take the DS register and add 1000h to it - segments starting at that adress are free to use for COM program.
However, you have to plan the allocation in your code and you cannot use DOS services to allocate memory.