The U210 is a Stand Alone rated plate. You don't need a backer to meet the tested protection levels. Adding a backer won't make it a Lvl 4 ICW because it hasn't been tested that way. I don't know if (but highly doubt) it would stop a black tip (AP) round within standards. Would need to be tested that way which costs a lot of money for official NIJ std testing.
However, a backer is a good idea as long as it doesn't add to much bulk or weight. Will provide additional blunt protection. I think (but have not tested) the U210 would stop .308 rounds within BFD limits with a backer.
I have LTC Stealth (28720) plates (.5 inch thick and 3.8 lbs). They are ICW rated plates but rated to stop all the same threats as U210 and also multiple hits of .308.
On a more regular basis, I also use Paraclete Speed Plate Plus (Tencate CR2000SA) plates. I use them with 3A armor underneath usually even though they are SA plates.