There are 43 students in a class, all of whom participate in extracurricular interest groups. Activities include musical instruments, painting, calligraphy and sports.

At present, A, B, C and D represent the courses of interest groups: A- musical instruments, B- painting, C- calligraphy and D- physical education. (This is done to simplify the following writing)

According to the meaning of the question, we can analyze:

There are four situations when you join an extracurricular interest group: (musical instruments) (painting) (calligraphy) (sports)

Namely a, b, c and d

There are six situations for each person to participate in two extracurricular groups: (musical instrument, painting) (musical instrument, calligraphy) (musical instrument, sports) (painting, calligraphy) (painting, sports) (calligraphy, sports).

Namely AB, AC, AD, BC, BD, CD,

There are three situations for each person to participate in three extracurricular groups: (musical instrument, painting, calligraphy) (musical instrument, painting, sports) (painting, calligraphy, sports). )

Namely ABC, ABD and BCD.

Everyone participates in four extracurricular groups 1: (musical instruments, painting, calligraphy, sports)

Namely ABCD

Then * * * has 4+6+3+ 1= 14 possibilities.

That is 43 ÷ 14 = 3... 1.

According to pigeon hole principle 2, there are at least 3+ 1=4 students.

In other words, at least four students in the class will add exactly the same project.