You can use the stock 4mm motor connectors that come on most motors without worry. They will handle the current just fine. Same with Deans Ultra connectors. Poor soldering techniques/skills such as low heat, dirty wire/connectors and cheap low tin solders causing cold solder joints are 99% of connector related failures not the size connector it self. In the past I've even used the bullets that come out of the Hobby King red connectors since I ended up with such a large collection over the years from replacing them with Deans.
Tom
