This is how it works.
The first drop down tells you how many vCPUs you want to assign for a VM.
The second drop down tells you if you want all the vCPU's you chose to be part of a single processor or different processors.
For Example:
If you choose 4CPU's and then say No. of cores per socket=4 then all 4 cores will be taken out of one socket(Which is what you need to do in your case since you have just one socket)
If you choose 4CPU's and then say No. of cores per socket=1 then each core will be taken out of 4 processor sockets(If you have so many that is).
Depending on the physical processors you have and logical cores they contain + the packing ratio that you decide, you will have to modify these values.
Hope i made it clear. let me know if you still have any confusions.