Do i need connect the 6LT first to my main computer and register every device or can can i do it in my new rig what im building?
What kind of "register" do you mean? It should work if you connect it to your new rig, then install the Aquasuite and activate the license.
Can i use a 4way splitter on my aquabus for 2x pump + 2x Flow? or do i need to buy 3 aquabus Y-Splitter?
What kind if 4way splitter do you mean?
As I wrote, you can use those 3 Y-Splitters or you could use a
Splitty9 for a kind of easy connection and also a easy way for upgrades on a later point of time.
Thats your choice in what you think would be easier for you, and also whats more affordable in price.