You’re not actually losing physical strength over a 2, 3 or 4 week break. You’re losing your ability to display the strength.
However, I do believe after 4 weeks you will begin losing strength, tied to loss of muscle.
This is why doing something is always better than doing nothing, provided you are not making your injury worse.
If you’re out of the gym for a low back injury, why cant you do back supported work?
I’ve been constantly injured or dealing with general health crap for 3+ years now. My strength is shit, but my physique has held tight or improved because I find work arounds. If I ever turn the corner, which I fully intend to, im confident I could build my strength again easily. As in, an aggressive linear progression run would have me at PR weights in no time. This is bc the muscle is still there, I just have to regain neural output.
Who cares if you lose some displayable strength, it’s only temporary. No one does this injury free for life if they have work ethic.
Keep training unless you are truly crippled.
Edit: when coming back from injury, the easiest method is to cut 30-40% bar weight and linear progress back up to your working weights. This allows you to use perfect technique in your comeback while also hitting some rehab/prehab work as well. Dont overthink it.