Is it possible to upgrade directly from Junos: 15.1X49-D60.7 to 18.4R3 or does it need to be step upgraded?
Yes it is.
"You can skip no more than two releases when upgrading (or downgrading). That means you can upgrade only to one of the three releases subsequent to your current release. If you want to upgrade across more releases than this, you need to perform multiple upgrades."
Valid SRX1500 versions:
You can go direct.... but ISSU is not supported when going from 15.1X49-DX to new major releases.
Notes for upgrading from Junos 15.1X49 releases to 18.4R3 or 18.4R3 based Service Releases:
Junos OS upgrade from 15.1X49 directly to 18.4R3 or 18.4R3 based Service Releases is supported for all SRX platforms.