Hello and welcome
Is the 40 full of water with an "ATBA" and what size turtle ?
For a guide a baby turtle need 2x's flow that of fish , juveniles 3-4 x's flow and adults 4-5x's flow. Also 10 gallons water for every inch of shell. Yes this is more swimming room but "mainly" to control the waters parameters so a filter can do it's job.
You also want a filter that does mechanical , chemical and biological filtering. Matching a filter to a turtle's size will only help.
A 406 is rated for 100 fish gallons ,//so if your turtle is about 3-4" it can work with 40 gallons for a turtle along with regular scheduled weekly maintenance and water changes. Yes any filter can pump water even in a 1000 gallon tank "BUT" will it be "adequate" to remove all the waste a turtle produces. A proper flow rate for a turtle goes a long way along with good media setup! Also helps with the amount of maintenance and you know how we all just love doing maintenance. Most filters come with media for fish don't be afraid to change it up more for a turtle. More bio media is your friend to hold a "cycled" tank! See this thread for some good ideas :
viewtopic.php?f=9&t=30145Here's a chart for some ideas :
http://www.redearslider.com/filter_chart.html