I think what you are talking about is a combiner box, like this one.
Free Shipping! MidNite Solar MNPV6 Combiner Box, 120A for 6 PV Breakers or 4 Fuses
ressupply.com
Does your Growatt have a breaker for the solar input? When you have multiple strings of panels, independent breakers are always a good idea, if only for troubleshooting.
BTW, I would suggest that 400W of panels is totally inadequate for a 48V system. What size/type of battery do you have? I'd recommend having at least 1000W. For my 48V system I have 4500W. Don't continue to buy 12V panels. They are very overpriced per watt. You'll get far better deals buying higher voltage grid-tie panels.