New Strain?

I know this is a stupid question and probably asked a million times... but in order to create a 'new' strain, do you pretty much just breed 4 different plants

AxB + CxD to ABxCD = E

please correct me if I'm wrong, I'm just a curious noob

A new strain per se would be as simple as Plant A x Plant B = new plant C provided that A and B were not of the same strain. This cross would be a f1 and totally unstable at this point. The real question is are plants A & B both stable enough to use in creation of a new strain. I say this because if you are using two f1s as your parents then you have much work and many many plants to grow out before you can make progress on actually having a new strain which will produce itself.
I believe on way that you can see how stable your parents are for breeding is to breed the 2 a few times and grow out the seeds to see if they are producing what you expected. 2 stable or for the sake of ease lets say 2 IBL or parents which are at least f4s breed to each other will produce the offspring time after time. The more variation within your parents the more phenos which are possible in your new seeds.
Hope this helps some, here are a couple of links you can read which will help you understand better.
https://www.rollitup.org/breeders-paradise/395655-creating-true-breeding-strains-vic.html
http://www.mellowgold.com/grow/mjbotany-removed/
https://www.rollitup.org/breeders-paradise/395659-mr-soul-brothers-grimm-cubing.html

Good Luck and Happy Growing

Thank you for the advice! I appreciate your breeding knowledge +rep