Let the output current of 24v voltage source be I 1, as shown in the figure.

For node 1:

[ 1+ ( 1/5)]? U 1 - ( 1/5)？ U2=I 1 ①

For node 3:

-( 1/4)? U2+[( 1/4)+( 1/4)]？ U3=-I 1 ②

Supplementary equation:

U 1-U3=24 ③

U2=4

Add equation ① and equation ② to eliminate I 1:

1.2? U 1+0.5？ U3= 1.8 ④

From ③ and ④, we can get:

U 1=8. 12 (V)，U3=- 15.88(V)

Therefore, I=(U2-U3)/4=4.97 (A).