Welcome to the Let's Encrypt Community, Jiang 
That's not technically accurate. All of Let's Encrypt's RSA intermediate certificates, including the legacy Let's Encrypt Authority X1...X4 and the current R3 and R4, have always had a version signed by DST Root CA X3 and a version signed by ISRG Root X1. What is changing is that the primary issuance for R3 and R4 will be from ISRG Root X1 while the alternative issuance will be from DST Root CA X3 with that alternative being phased out as the expiration of DST Root CA X3 (Sep 30 14:01:15 2021 GMT) approaches.
Perhaps this may help?
This may be of use too: