A B.P. scalpel handle No. 3 & 4 combined refers to a single instrument that has two ends, each designed to hold a different size of scalpel blade:
 
No. 3: This is a commonly used size, suitable for various surgical procedures.
 
No. 4: This is a larger size, typically used for deeper cuts or dissections.
 
This combined handle offers the advantage of having both sizes readily available without needing to switch between separate handles during surgery.
 
Here's some additional information you might find helpful:
These handles are typically made of stainless steel for durability and ease of sterilization.
They are reusable but require proper cleaning and disinfection after each use.
You can find B.P. scalpel handles No. 3 & 4 combined for sale from various medical supply companies [search online for "B.P. scalpel handle No. 3 & 4 combined"].