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However, they are not equal, and in this new day and age, there are many other factors, such as the limiting of pagerank passed over time, the ability of Google to prevent pagerank passing, and many other factors.
I won't say that pagerank is not important - more that is the result of a well promoted website.
In this day and age I would be shooting more towards traffic and conversion than looking at that specific number.
Has you overall website traffic increased over the 3 years? If so, that is definitely a good KPI to congratulate yourself for.
Is the gap a lot bigger than going from 3 to 4 PR?
The PR scale is approximately logarithmic, with each number or step being around six times as intense. And even though PR is displayed on the toolbar as eleven possible numbers (plus no number at all, or a gray bar) Google actually calculates PR to many decimal points on their back end.
So odds are, if your back links are growing - and especially if they are growing faster than your competition's - you are not "stuck" at PR4. Instead you're growing from 4.00001 to 4.20003 to 4.58223, etc.
PR is sooo 2003 ;)
I accidentally discovered yesterday, and confirmed a few minutes ago, that a 13 year old domain name, superceded for 10 years and entirely 301-redirected to another domain for 6 years has a PR of 5. I doubt it's anything to do with back-links - google only gives about 35. On the other hand, the site it redirects to also has PR 5, also with 35 links.
Mind you, the primary domain does come in the top three for at least two of its key phrases. Which is surprising, as no changes to the site have been made in five years, it's a framed site, and well past time it was rebuilt.