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:

1. 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.

2. Collagen, one of the main structural proteins in connective tissues in the human body, is well known for promoting skin regeneration.

3. Fish skin swims in here as diseases and bacteria that affect fish are different from most human pathogens.

4. 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.