So, you know the film and you know what ISO it was exposed to. It is not big deal then as long as you know where to search.
https://www.digitaltruth.com/devchart.php?Film=%Ilford+HP5%&Developer=&mdc=Search&TempUnits=C&TimeUnits=D
And, no, it is better not to develop it as usual. Exposing 400 film as 100 is called "pulling" and it requires different developing.
Pulling requires less developing, pushing needs more. If you or else will develop it as usual, it will gives darker than usual negatives. because they will be overdeveloped.
And here is the answer for how it looks like:
https://www.flickr.com/groups/32592441@N00/discuss/72157605596327500/
BTW, If you could make it to Burlington Camera (in Burlington) they are people who still develop film and used to do it for decades as business. I always ask them for advice. You could get chemicals here and not expensive used developing tanks and darkroom enlarger and other accessories. Not expensive.