How to delete all the commits before a specific commit in Bitbucket

Question

I've a project in Bitbucket feed via git Bash with too much commits and I want to conserve only the 5 last commits and delete all the commits before. For example for the next sequence

Last commits:

            6546464
            3684636
            3684368
            5785447
            3485888
            5784361
            6843684
            3878466
            3438438

Oldest commit: 3843838

I want to save only the newest commits 6546464, 3684636, 3684368, 5785447, 3485888 and delete all the others committed before (can be hundreds)

How to do this clean action with git bash?

Thanks