Hello, i do have a 40 gallon aquarium and a 20 gallon sump at the bottom of my stand. The height of my tank from the sump is 4 feet. can you advise me in what flow rate do i need fro this kind of setup?
Many ways to do, but my suggestion would be to combine a small sump pump with one or more small power heads internal to the main tank. Typically if you do not have to rely on the sump pump for all your circulation needs you can get by with a pump that turns over you tank only 2-3 times an hour to allow proper temperature control (assumes heater in sump) and to supply any additional filters operating from the sump. For your 40 gal tank that means about 120 gal/hr, allowing for a head loss of say 100% (will highly depend on pump type but this should be sufficient for most pumps with at least a 8-10' max head or so) you need about 240 gal/hr or more. Adding some additional flow rate capability that you can always valve down if needed, I would round this up to say a 300-350 gal/hr sump pump (external or submersible).
You can then add one or more (will depend on flow requirements of planned tank inhabitants) smaller power heads inside of the tank of say 150-200 gal/hr capability each and get the advantage of more flexibility in setting up/adjusting circulation flow patterns in the tank (especially should you decide to add wavemaker) as well as the added reliability that multiple pumps bring to your system. About the only equipment failure likely to adversely effect the tank quicker than no circulation in tank due to pump failure is a failure of the tank itself.
A bit more complex than you may have had in mind but I think more likely to help with your long term tank success than a single sump pump would.
300-350 gal/hour with a minimum max head of 8-10' should be OK if you supplement it with additional internal circulation pumps (e.g., power heads) as suggested.
If you are constrained to use only one circulation pump for the sump/tank then you probably want to double this size to 700 gal/hr or so to allow up to 5-6 times turnover rate/hr in your tank (you can always reduce the flow rate with a valve if you feel too much). Your actual circulation requirements will depend on what you plan to keep in your tank but this recommendation should work with the more common setups.
If only using one pump you may wish to consider splitting the return to the tank into two or more lines so as to both reduce head loss in the pump as well as give added flexibility to control circulation patterns in your tank.
Remember the quality of the circulation in the tank (i.e., avoiding stagnant flow patterns) is just as important as the total flow rate.