Shooting at a 4'x4' target at 900m/1000y is doable, if you have everything set up and figured out already. If you don't, or if you are doing any sort of load development, using a 6'x6' target board is a vast improvement. Standard US practice is to use 6'x6' target boards for long range (800y-1000y) shooting. In Canada we use 6'x6', or 6'x8', and occasionally even 6'x10' (all are 6' high; the larger dimension is the width, which can help keep bullets on the target board in variable wind conditions).
For shooting with iron sights at 1000y, you need at least a 6'x6' light coloured board, with a reasonably large dark aiming mark (the US target is a 44" diameter black, the Canadian and British is 48").
Using a scope makes things a lot simpler. All you need is an aiming mark that provides enough contrast to give you a decent sight picture. A darker coloured circle or square that is 1' or 2' in size would be just fine. You might go to a local newspaper office and get their newsprint ends - staple that onto your target board to make a clean surface in which shot holes will be visible, then put an aiming mark in the middle. As you shoot your target, either mark your shot holes with a pen or put a patch (or piece of masking tape) over the holes.
