The four sentences (labelled $1, 2, 3$ and $4$) below, when properly sequenced, would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer:
- Fish skin collagen has excellent thermo-stability and tensile strength making it ideal for use as bandage that adheres to the skin and adjusts to body movements.
- Collagen, one of the main structural proteins in connective tissues in the human body, is well known for promoting skin regeneration.
- Fish skin swims in here as diseases and bacteria that affect fish are different from most human pathogens.
- The risk of introducing disease agents into other species through the use of pig and cow collagen proteins for wound healing has inhibited its broader applications in the medical field.