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To answer your questions: a 5x8 will work. You can make anything work if you want it bad enough.
You don’t need a buffer tank (water tank) for a 4GPM.
Also are you using a direct drive or belt/gear drive? A direct drive won’t pull water efficiently. Some of them WILL pull enough water to use a buffer but in general they vibrate a lot reducing the ability to pull water efficiently. A booster pump is recommended by manufacturers for all 4GPM direct drives if used with a buffer tank. Later when you upgrade a 5.5GPM can go as low as 35 gallons but I wouldn’t go below 65 gallons myself. For an 8GPM I wouldn’t go below 100 gallons (personal preference here).
