Your welcome and of course in the spirit of DIY there is no charge.
Yes 4 AWG would work given that each Growatt can only draw about 60A but I would feel better with some extra margin but if you have the cable on hand wire it up and at least get a feel for what your doing and then you can swap the cable out at a later date.
300 Amp Busbar is more than enough.
I just did a quick search and saw this one, but please look at the reviews and see what is the best quality you want to buy.
Amazon Busbar
Edit:
Just noticed I did not label the two negative wires that go up to the Inverter. Those must be the same length as E&F / G&H. So if E&F add up to 7ft then G&H must be 7ft and the two negatives must be 7ft. Always look at what is going to be the longest wire and make that first then make the copies to match the length.
I am going logging out now for several hours so good luck.