What is the next step?

I started playing chess 4 days ago,  played more than 50 games and done more than 100 puzzles.

So, I know the basic rules of chess, but I want to improve more.

I grabbed a book called "Chess Tactics For Students" it's pretty much easy to understand, but I would love to go beyond just the basic tactics introduced in this book.

What do you suggest to be my next step, should I stick with tactics and learn more about them (any book recommendation will be appreciated), or play more games, or start learning about openings?

IMO the 5 basic areas are: openings, strategy, tactics, endgames, and annotated game collection.

In the beginning studying a specific opening in depth is overkill. It's enough to know the opening principals and use a free online database to check the most popular opening moves after you play a game.

For study it's better to focus on any of the other areas. In particular new players often win and lose games based on tactics and observation (is the square I want to move my piece to safe? Was my opponent's last move safe?).

So I recommend 1 tactic book plus one book on something else. A game collection is good because it gives you a little bit of knowledge on everything.

Tactic book
https://www.amazon.com/Back-Basics-Tactics-ChessCafe-Chess/dp/1888690348

Game collection:
https://www.amazon.com/Masters-Chessboard-21st-Century-Richard/dp/1936490218

Many thanks, sir.

I increased the level of computer's difficulty up to 2, and I captured all of the pieces easily while the computer was making stupid moves. by time when I had more than 12 pieces against the computer's king, I felt like I'm the stupid. every move results in a stalemate. is that an end game or a tactic mistake?

For your 2nd book, I recommend Chess Fundamentals by Capablanca. IYou won't regret buying and studying it.

Chess Fundamentals is for intermediate players.. An excellent one containing a little of everything.