Sigh... here we go again with the "remove your crushed coral" crowd. Will you please stop? You can keep anything you choose to keep. Sand, crushed coral, or bare bottom... they all can work in a tank and all have differing strengths and weaknesses. Instead of telling someone how to properly set up the tank they have, some people are telling them to set up the tank they have. If you keep crushed coral less than an inch deep and have a good cleanup crew it will be fine in a tank.
Here's a picture of my awful crushed coral tank. I haven't cleaned it in at least a year.
It would be a lot easier if people didn't have to fight even the some of the moderators over this one... as they seem to join in the "crushed coral is always bad" crowd as well instead of telling people how to properly set it up if that's their choice. IMO, this is the single largest disservice SWF.com does to the new hobbiest. The most common advice regarding substrate is "scrap everything and do it like I do".
Oh, btw... I've been snorkeling all over the Caribbean and Hawaii. Guess what is at the bottom of most natural reefs? Crushed coral. It's more common than people think. I'm sorry for ranting... it's just that this seems to come up quite a bit and people end up spending lots of money scrapping what they have instead of being told how to make it work.
As for your original question, you are only on your 4th week of cycling. If you are still showing ammonia and nitrites then you don't need to be adding anything to your tank. If you have any prefilters in your system, clean them out so that they won't add contamination to your tank. Do some partial water changes to lower your levels down and then see if they stay down. If they do then you are ready to add a few things... but slowly. Only add one fish per month or so as your tank will need time to adjust. If you want to switch over to sand then yes, it will interrupt your cycle some. Do a search for southdown sand. Aquarium grade sand is different from just ordinary play sand in that it does not release large amounts of silicate into the water and this helps to reduce algae. Lowes/Home Depot sell this kind of sand, or so I've heard.