As @grb already stated, the announced “phasing out” doesn’t affect the Yubico OTP option. Here the corresponding statement from @Micah_Edelblut : FIDO U2F keys are being phased out in 2025 - make sure to replace those in time - #34 by Micah_Edelblut
The “phasing out” only affects the “passkey”-2FA-option (formerly called “FIDO2 WebAuthn”, formerly called “FIDO U2F”).
And though I’m not completely sure, I think you could use your YubiKey 4 with the “passkey”-2FA-option. Though the key supports “only” U2F and not FIDO2, Bitwarden would still register it with the newer WebAuthn protocol… 