Beyond that, it's better to follow the best practice to not hop more than 3 Junos releases. So i'd suggest the safest way forward:
15.1X53 ---- > 17.4 ----> 18.3 ----> 18.4
If you have an option of USB install, a format install will work directly. But will need to rebuild the configuration. If USB is an option, then it might be best if you upgrade one device with step upgrades and ensure its smooth with config updated and use it to get your 18.4 config template. Then upgrade the rest with USB, and use the config template (18.4). Just a thought!