Hello!

I am kinda confused regarding when to use 2 probe and when to use 4 probe. I am pretty sure 4 probe measurements eliminate the contact, probe and spread resistances. It would be useful in measuring 4 terminal device. However what if i have 2 terminal device such as pn-diode and solar cells? do I use 4 probe or 2 probe? Can someone explain this for me? thank you in advance!

 

Four probes have nothing to do with contact, probe or reducing resistance.
Four probes have nothing to do with 2-terminal devices or 4-terminal devices.

What is the difference between using a single channel oscilloscope and a 2-channel oscilloscope?
It is like having two voltmeters instead of one voltmeter.You can make two measurements at the same time.
A 2-channel oscilloscope is better than having two separate oscilloscopes. That this because the two measurements on the same scope can now be synchronized. (You could do the same thing with two oscilloscopes if you triggered both scopes from the same signal or synchronized signals.) Also a 2-channel oscilloscope (a modern one) allows you to take the sum or difference of the two signals and show the answer directly on the screen.

Hence, one probe allows you to see one signal.
Two probes give you two signals
Four probes give you four signals.

If you do not know how to take advantage of two probes then you do not need four.

Edit: Sorry, I misunderstood the question. I now assume that you mean 2-terminal versus 4-terminal measurements as in measuring electrical impedance.

Here is an answer:
http://www.tek.com/sites/tek.com/files/media/document/resources/2Wire_4Wire Resistance Article.pdf