Size issue

I recently purchased Nike court vision online, and I loved them at first. However, despite ordering my usual size 4, I noticed that my foot slightly slips out while walking, which is annoying. Unfortunately, size 3.5 is not available, and I am sure that size 3 would be too tight. While the shoes are wearable, I've heard that they may stretch with frequent use. Should I return them? Or any tips to fix this issue?