ones

Yes, 1 and 2 mean the same thing. You could substitute persons, individuals, or human beings for 'ones' without changing the meaning. 'People" is the most commonly used term.

4 is incorrect. 'One' is singular and requires the singular noun 'life'.

He was thinking about the brave man who had given his life for a good cause.

or

He was thinking about the brave woman who had given her life for a good cause.

Using 'man' or 'woman' avoids confusion. If you use the term 'one' it was customary to use 'his'. Lately this has become controversial. Some people prefer 'his/her' and some people use 'their'.