Having capital letters in your URL is perfectly fine. It doesn't affect the performance of your website and shouldn't cause any problems for search engines.

I can only guess that whomever told you it could cause 404 errors meant that if someone typed in the URL manually and didn't use capital letters that the page would not be found. That's true but the reverse is also true (if someone uses capital letters and your URL has none) so that's hardly useful information. But if you are concerned about this affecting your users you can always use mod_speling (yes, one L) to allow your URLs to be case insensitive. As far as it affecting search engines, since they follow links and sitemaps they won't be using the wrong URL unless you tell them to.

Having capital letters in your URL is perfectly fine. It doesn't affect the performance of your website and shouldn't cause any problems for search engines.

I can only guess that whomever told you it could cause 404 errors meant that if someone typed in the URL manually and didn't use capital letters that the page would not be found. That's true but the reverse is also true (if someone uses capital letters and your URL has none) so that's hardly useful information. But if you are concerned about this affecting your users you can always use mod_speling (yes, one L) to allow your URLs to be case insensitive. 

As far as capitalization affecting search engines, since they follow links and sitemaps they won't be using the wrong URL unless you tell them to.