CT-ART 4 is a tactics training program by ChessOK.
It contains 2,200 basic exercises and 1,800 auxiliary exercises in total.
You can solve tactics by theme (23 different themes) or by difficulty level (9 levels). You also have the option to test yourself. The test gives you the same problems as in the practice mode but in a different order. At the end of the test, you have statistics such as the percentage of correct answers.
In the description on the chessOK site, they say that CT-ART 4 is for players from 1200 to 2200 elo. I am rated 1588 FIDE and I find level 2 challenging with only 80% of correct answers. This may be because tactic is one of my biggest weaknesses. Level 1 was fairly easy. I’ll go to level 3 once I have mastered all the problems in level 2, because the gap between levels seems big.
The thing I like most about this program is that if you don’t find the correct answer, you are given an easier version of the problem to solve. Then you can work out the solution to the harder problem.
One of the big pros is that since they released CT-ART 6, the price of CT-ART 4 became really affordable (€12.76 / $15.95).
The download link delivery was instant and the installation was easy.
In conclusion, I would recommend this program to any club player who wants to work on his tactical vision. It is cheaper than most tactics books and it is easier to use.