If it came to relying on luck, then fusing four Waist Bags +4 into one Waist Bag +5, although the success rate was very low, was actually still possible. However, Zhang Yanhang didn't really want to gamble on such things, because if the fusion failed, all four of his Waist Bags +4 would be rendered useless.
If all those Waist Bags +4 were to become useless, then all Zhang Yanhang would have left was a single Waist Bag +1 that he could freely use. At that point, it would truly be a genuine case of back to square one, which would be a real tragedy. He would have only four slots to use, and out of those, he would still need to free up two slots, or at least one slot, for a Storage Box, further reducing his efficiency.
If that really happened, Zhang Yanhang might have to resort to using the bags made from old clothes for storing items again, and the retired family heirloom shoulder bag would also have to be re-employed and continue to work overtime—it was a miserable situation indeed.