np.random.seed(100)
df1 = pd.DataFrame(np.random.randint(100, size=(5,6)), columns=list('ABCDEF'))
print (df1)
    A   B   C   D   E   F
0   8  24  67  87  79  48
1  10  94  52  98  53  66
2  98  14  34  24  15  60
3  58  16   9  93  86   2
4  27   4  31   1  13  83

df2 = pd.DataFrame({'A':[1,2,3],
                   'B':[4,5,6],
                   'C':[7,8,9],
                   'D':[1,3,5],
                   'E':[5,3,6],
                   'F':[7,4,3]})

print (df2)
   A  B  C  D  E  F
0  1  4  7  1  5  7
1  2  5  8  3  3  4
2  3  6  9  5  6  3

print (pd.concat([df1.iloc[:4], df2, df1.iloc[4:]], ignore_index=True))
    A   B   C   D   E   F
0   8  24  67  87  79  48
1  10  94  52  98  53  66
2  98  14  34  24  15  60
3  58  16   9  93  86   2
4   1   4   7   1   5   7
5   2   5   8   3   3   4
6   3   6   9   5   6   3
7  27   4  31   1  13  83