Disclaimer: I don’t have a Fairphone 4 myself, I can only do my best going by logic, by things which are the same across devices as well as by what I can cite as a source.
No. As the instructions say that the patch level of /e/OS needs to be newer (or “greater”) than the patch level existing on the device before installation to be able to lock the bootloader, there’s no safer choice than simply using the latest /e/OS image.
The security patch level indicates how recent the security patches in the OS are. Google patches the Android base for all Android OSes monthly, the many different Android OS vendors including Fairphone with Fairphone OS and the e foundation with /e/OS then apply those patches to their own OSes within their own update schedule, more or less timely.
So, across OSes the existing patch level on a device not only depends on when a user updates the OS, the patch level really can differ between different OSes at the same point in time.
Like right now for instance. The latest Fairphone OS update for the Fairphone 4 as of right now has security patches as recent as 2022-05-05 (see above), the latest /e/OS updates for the Fairphone 4 as of right now have security patches as recent as 2022-04-05 according to the install instructions.
How dangerous it is depends on your personal risk assessment in your very own mindset and circumstances.
User data should be inaccessible after simple loss or theft of the phone as long as you chose a PIN/password/pattern for your screen lock, which will automatically serve as the necessary PIN/password/pattern to decrypt user data. But who really knows? Software these days can’t guarantee anything other than that it will have bugs at least, if not conceptual shortcomings 
.
With an unlocked bootloader anything can be booted or installed by somebody getting physical access to your phone. So somebody snatching your phone unnoticed, installing malware to grab your unlock method or to do stuff with your data after you unlocked, then slipping the phone back to you still unnoticed just to wait for you to unlock it … is a possibility for you to either prevent or accept.
Thats a reasonable expectation, but in practice delays can happen due to technical difficulty and whatnot, so don’t fix yourself too tight to an exact date … or week 
.
See here … and be aware that locking (as well as unlocking) the bootloader will force a factory reset for security reasons …