I’m just a technician not an installer, so my product knowledge is low. Sunny Island is about $5k for the 4.6kW model on eBay and about $6k for the 6kW model. So $4k looks like a really good price. Looks like Sunny Island run a 48V battery bank, so I imagine that’s your chosen battery bank voltage.
Victron Energy (since 1975) do their 48V 4500W MultiPlus inverter/charger which is on eBay for $3.3K. It is intelligent enough to manage one AC input, say grid or generator.
Outback Power do a 48V 3kW for $3.2k on eBay. I think Selectronics 48V inverters begin at 5kW, so are more expensive.
Only an installer could probably answer your questions, as it depends on your current solar system setup and what type of solar system you want to expand into for the future. e.g. how many buildings, PV strings etc. The inverter/chargers are all slightly different around how further system architecture is added. The other challenge is some installers only know how to install the gear they use, so I’ve found its good to get quotes and ideas on system design from each. The solar quotes website that often has links on this website looks good for getting ideas on planning systems.
if your considering the LG Chem for storage, http://www.bvim.com.au say they match the LG Chem to the SMA Sunny Island 8.0H/6.0H Sunny Island 3.0M/4.4M or Sungrow Hybrid Inverter. They are the training org for installers of LG Chem. I haven’t read the manuals to see if these really are the only inverters that can be connected.