Here, it is similar: you have 32 bytes for R-value of ECDSA signature, and you can store everything behind it, including a quantum resistant 50 kB signature, for each and every R-value you currently have, when OP_CHECKSIG or its equivalent is called in the Script.
I don't understand how ECDSA and the R-values comes here into play. I think "block size" need to be increased to accommodate larger signatures. 4 MB is not enough at all. 4 MB can only include approximately 70x transactions with SPHINCS+, while currently there are 3-5000 transactions in a block. See what I described previously:
https://bitcointalk.org/index.php?topic=5553484.msg65677531#msg65677531