Mining should be available to everyone, regardless of their access to specialized hardware; anyone should be able to use a computer to “mine” by using open source software, and add more transactions to the Zcash ledger to possibly get Zcash coins in return for their effort
https://zcashminers.org
Mining solo with 4.5 Sol/s. How does this fit into the above statement?
Technically you can mine with 4 Sols/s but your likelihood of winning a block mining by yourself is based on the total network difficulty. Right now the difficulty is around 750,000 so your best bet to have your mining go towards the accumulation of Zcash is to join a pool.
4.5 sols is nothing more than a older laptop cpu. My old i3 1.3ghz did about 3sols. Keep it running for a year on a mining pool and you'll have about 0.1 zcash at the end of 2017. If everything keeps decent stable u'll maybe have about half a zcash when the last block is mined.
Could probably be improved with better cpu mining SW, I personally think the founders should have developed some mining SW or paid some developers to do so as they're charging 20% of the mined blocks. That would be in their own interest as more diversity is better for zcash. But it is what it is. Develop a better CPU miner urself if ur unhappy about the results 
Does joining a mining pool centralize the network? I'm assuming miners in a pool share a node or a couple nodes.
How much does mining support the network? Will running a node without mining support the network?