I can understand how wanting to simplify cleaning your filter would be a good selling point for one of these Fluval U4's but everything I have ever read just says these internal filters just cant keep up with a turtle tank.... no matter how much they say it can. With a 60 gallon tank, assuming you are filling it up 2/3 to 3/4 of the way full of water its still filtering 40-50 gallons of water. But as you may have read water gallons are not the same as "Turtle Gallons" where as each turtle gallon is equal to 3 water gallons. In short you need 3 times the filtration for how ever much water you are filtering. The 405 is really only sufficient for 40 turtle gallons. This might explain why its taxing your filter so much. The U4 is for aquariums up to 34 to 65 US Gallons according to the manufacturers site. Again that's Water Gallons, not Turtle Gallons. For your 60 gallon tank the experienced folks here have put together a list of suggested filters. (found at this link
viewtopic.php?f=9&t=30145 They include
Fluval FX5: Up to 150 turtle gallons
XP3 aka API FilStar XP Large: Up to 75 turtle gallons
XP4 aka API FilStar XP XL: Up to 100 turtle gallons
Eheim Pro III 2075: up to 75 turtle gallons
Eheim Pro IIIe 2078: up to 90 turtle gallons
Eheim Pro III XL 2080: up to 150 turtle gallons
Eheim Pro III XL 2180: up to 150 turtle gallons
Eheim Classic:
2260: Up to 130 turtle gallons
2262: Up to 150 turtle gallons
JBJ Reaction
EFU-45: Up to 75 turtle gallons.
Penn Plax Cascade
CCF5UL aka 1500: Up to 75 turtle gallons
If you were going to get the U4 I would almost suggest using it in together WITH your 405.
The U4 would require 3 of them to be set up in a 60 gallon tank to provide proper filtration.
Hope this helps