Google Cardboard with 4.5 inch smartphone
I won't go for it on a phone with a 720p display, at least 1080p is a must for Virtual Reality. The 4.5 inch phone will work and fit, just pick out the right cardboard. I'd recommend the Moto G3 with 1440p for VR, its actually perfect, your phone is a bit out of date. But if its all you need in a phone, don't bother switching.