The best way depends on your situation.

But in general the best thing would be to create your own module, if you want to extend an official module.

If you have downloaded a module from internet, and you had to upgrade it to match V7, than you might want to use that module. But even then, making your own module would be preferred.

Making your own module(s) and extend other modules, means you have all your modifications in 1 place, you can use your new fields to do things with (for example calculations), when you install a new database, you just install your module(s) and everything is there. And you can share your modifications with the rest of the world, without the need to add additional information about what field to create where and with what name.

Option 1 of your question is something you can do to quickly add a field, but you can only fill it, and print it on forms. Disadvantage is that you have to do everything manually (easy to forget when making a new database).

Option 4 is sort of the same as option 1.

Option 2 is better, because of the reasons above.

Option 3 is the best, given the reasons above.