It is more important that you have unrestricted flow into the cistern.
Cistern no gutters.
Screwed connections are stronger.
Half round gutters rest in brackets screwed to the fascia.
I chose to use 2x6 material because i had it handy but if you choose to u.
This is a cistern i made from some pressure treated 2x6 material and a 55 gallon plastic juice drum.
Sweep all debris from the roof and gutters leading to the chosen downspout by using a broom.
Install standard size mesh gutter covers over the entire gutter that will lead into the cistern.
Traditionally this involves harvesting the rain from a roof.
Another method calls for gutters to be held in place by straps fastened to the roof.
I got the used drum locally for 15 and the wood came from my scrap bin.
This technique is usually only used when there s no fascia board.
Remember that the x section of a 4 pipe is twice that of a 3 pipe.
K style gutters can be spiked or screwed to the fascia.
If you join too many downspouts before they enter the cistern you could have a constriction that would be problematic.
Cistern covers are 2 x2 and are cast in the floor.
The rain will collect in gutters that channel the water into downspouts and then into some sort of storage vessel.
A system of gutters and downspouts directs the rainwater collected by the roof to the storage cistern.