Hi,

I hope it is okay for you, that I do comment your question here because it shows a problem the J51 team has atm..... (btw: I am only a web developer, no J51 team member ^^)

I checked the source code of your website and yes, you are using j51_london.

If you want to upgrade to J4 maybe you might think about using another template of J51 for your site?
E.g. J51_Jasmine could be a possible template. It has nearly the same structure like London, the module positions are the same as on London and if you have done all your customisation in a custom.css you only need to place the custom.css on media/templates/site/j51-jasime/css instead of templates/j51-london/css.
The advantage would be that you have a newer template with better coding, better features than in the very old London template.
Only an idea

If you need help, doing this, let me know. Maybe I could help you with this.

To come back to my first sentence: the reason why the J51 guys don't do any new templates is that they do upgrade all the old and very old stuff. This will last to the beginning or maybe to the mid of next year.

Kind regards
Oliver