Is the film holder inserted all the way into the gate?
Does your enlarger have a "beam spread" adjustment used to optimize the lamp position for different film formats? (e.g. You raise it up for 35mm and lower it for 120.)
Even if your lamp is out of adjustment for the film you are projecting, there still shouldn't be any stray light getting by the film holder. You would just have vignetting or a hot spot/
There shouldn't be any light getting around the negative. The film holder blocks that from happening unless it's not fitted properly or if the gate isn't closed all the way.
What if you got a large piece of cardboard, cut a hole in it just big enough for the lens and held it up to the bottom of the enlarger? Would the marks still be there?
That would prove whether the light is or isn't coming out of the lens.
I'm a little confused by this... You say that there are four sharply focused rectangles bordering the image? Right? Sharply focused?
If they are sharply focused, they couldn't be just any old stray light coming from the enlarger.
I can think of two things would cause a sharply focused edge:
1) Something AT THE FILM PLANE.
2) Something ON THE EASEL.
The blades of the easel, lying flat on the paper, would make any stray light look sharply focused. The only other thing I know that would make sharp edges would be something at the film plane that's letting light through which is being focused by the lens. Even an internal reflection from the lens is likely to be blurry. (Unless the easel blades sharpen it.)
And, at the risk of sounding like a pain, are you sure you are using the right film holder that has the right sized hole in it which matches the size of the image on the negative?
If the film holder doesn't match the size of the image on the negative... if the hole in the holder is too large... you will have light leaking around the negative and it could possibly be sharply focused.
More questions: You are absolutely sure your paper isn't getting fogged by room light or your safelight?
Are you using a red "safety filter" under the enlarger to focus the image? (Or some setting of the enlarger?)
I never do that. I always sacrifice a sheet of paper (fix, wash and dry without exposing it so that it's pure white) to put under my enlarger to focus on. I never take my paper out of the box until I'm ready to burn it.
Are you sure that there is no stray light in the room? No gaps in the safelights? No holes in widow coverings or doors?
You're not turning on a light in the room or using a flashlight? (Even if it's filtered red.)
You are using a four-blade easel. Right?