Bundler compatible versions for gem "jekyll" – SEO

I’m trying to setup github page using Jekyll, currently it is using minima theme, and I want to change it to jekyll-theme-hacker.

So, I went to Gemfile,
I added

gem "github-pages", group:jekyll_plugins

under the

gem "minima", "~>2.5"

And after saving it, I did

bundle install

But this gives me the error.

Bundler could not find compatible versions for gem "jekyll":
  In snapshot (Gemfile.lock):
    jekyll (= 4.0.0)

  In Gemfile:
    jekyll (~> 4.0.0)

    jekyll-theme-hacker was resolved to 0.0.4, which depends on
      jekyll (~> 3.3)

Running `bundle update` will rebuild your snapshot from scratch, using
only
the gems in your Gemfile, which may resolve the conflict.

I am very new to the Jekyll and Programming, I wonder why this happened.

As far as what I could figure out was, Bundler is a Gem management tool, which Gem stands for the library or module for ruby programming langauge. So what I was trying to do was trying to install all the necessary things for the my new theme.

However bundler could not find compatible versions for gem “jekyll”.
What does this exactly mean? Do I need a different dependency of jekyll to use my new theme?
If it is so, How can I do it?

Thanks a lot in advance.

—-EDIT—-
This is how my Gemfile.lock looks like

GEM
  remote: https://rubygems.org/
  specs:
    addressable (2.7.0)
      public_suffix (>= 2.0.2, < 5.0)
    colorator (1.1.0)
    concurrent-ruby (1.1.5)
    em-websocket (0.5.1)
      eventmachine (>= 0.12.9)
      http_parser.rb (~> 0.6.0)
    eventmachine (1.2.7)
    ffi (1.11.1)
    forwardable-extended (2.6.0)
    http_parser.rb (0.6.0)
    i18n (1.7.0)
      concurrent-ruby (~> 1.0)
    jekyll (4.0.0)
      addressable (~> 2.4)
      colorator (~> 1.0)
      em-websocket (~> 0.5)
      i18n (>= 0.9.5, < 2)
      jekyll-sass-converter (~> 2.0)
      jekyll-watch (~> 2.0)
      kramdown (~> 2.1)
      kramdown-parser-gfm (~> 1.0)
      liquid (~> 4.0)
      mercenary (~> 0.3.3)
      pathutil (~> 0.9)
      rouge (~> 3.0)
      safe_yaml (~> 1.0)
      terminal-table (~> 1.8)
    jekyll-feed (0.12.1)
      jekyll (>= 3.7, < 5.0)
    jekyll-sass-converter (2.0.1)
      sassc (> 2.0.1, < 3.0)
    jekyll-seo-tag (2.6.1)
      jekyll (>= 3.3, < 5.0)
    jekyll-watch (2.2.1)
      listen (~> 3.0)
    kramdown (2.1.0)
    kramdown-parser-gfm (1.1.0)
      kramdown (~> 2.0)
    liquid (4.0.3)
    listen (3.2.0)
      rb-fsevent (~> 0.10, >= 0.10.3)
      rb-inotify (~> 0.9, >= 0.9.10)
    mercenary (0.3.6)
    minima (2.5.1)
      jekyll (>= 3.5, < 5.0)
      jekyll-feed (~> 0.9)
      jekyll-seo-tag (~> 2.1)
    pathutil (0.16.2)
      forwardable-extended (~> 2.6)
    public_suffix (4.0.1)
    rb-fsevent (0.10.3)
    rb-inotify (0.10.0)
      ffi (~> 1.0)
    rouge (3.11.1)
    safe_yaml (1.0.5)
    sassc (2.2.1)
      ffi (~> 1.9)
    terminal-table (1.8.0)
      unicode-display_width (~> 1.1, >= 1.1.1)
    thread_safe (0.3.6)
    tzinfo (1.2.5)
      thread_safe (~> 0.1)
    tzinfo-data (1.2019.3)
      tzinfo (>= 1.0.0)
    unicode-display_width (1.6.0)
    wdm (0.1.1)

PLATFORMS
  ruby

DEPENDENCIES
  jekyll (~> 4.0.0)
  jekyll-feed (~> 0.12)
  minima (~> 2.5)
  tzinfo (~> 1.2)
  tzinfo-data
  wdm (~> 0.1.1)

BUNDLED WITH
   2.0.2