I had a Iphone 4 before the FP2 and its hardly comparable. The FP2 (143x73x11) is already bigger than the Iphone 4 (115,2 × 58,6 × 9,3). The FP3 (158x71.8x9.89) is different (more height, less width) compared to FP2 and the FP4 (162x75.5x10.5) is the biggest Fairphone.
I loved the measurement of the Iphone 4 as well, but adopted fast to the FP2 and I’m sure would adopt as well to the Fairphone 4, although I assume for me it would be difficult to handle it single-handed due to the width.
I would suggest to check if there is a community close, so that you probably hold the phones in hand before you decide to order one