Most pairs in the Yi are reflections of each other. Like a mountain in a lake: upside down.
Another combination is the 'contrast': every yang line in one hex is a yin line in the other and every yin line is a yang line.
When you cast hex.3, the Yi gives advice. But 4 does have some meaning in the advice as well. It is the view from the other side. Not advice about what you should do now, but sometimes it is helpful to see "the other perspective".
The top is now at the bottom, the bottom at the top. Line 3.1 becomes 4.6, and so on. Line 3.5 is about finding a good place. Its reflection, 4.2, is also about a time without solid facts, but now about caring for what is present already.
Hex.3 line 5 changes to hex.24, hex.4 line 2 changes to 23, which is - of course - the reflection of 24, because that is how it works.
Between 3.5 and 4.2 is a reflection. For hexagrams and for lines it works the same. In hex.3 there is a beginning without anything already finished.
In hex.4 the start is present, but you don't know yet how to proceed.
Starting or learning - two sides of a similar dilemma. In hex.3 you have to start acting. In 4 you have to learn how to act.