One thing that sticks out to me more than anything in ch 4, is the use of the word "sister, bride" to describe his lover.
There is a respect and a deep love beyond just the physical. By identifying her as a sister, he is embracing her as a companion, by calling her his bride, he shows she is someone he admires and respects. Not only is it a deep respect but a passionate love surrounding that respect.
You have stolen my heart, my sister, my bride; you have stolen my heart with one glance of your eyes, with one jewel of your necklace.
How delightful is your love, my sister, my bride! How much more pleasing is your love than wine, and the fragrance of your perfume than any spice! **
**Song of Songs 4:9-10