Four inches high at 100 is usable, depending on what it is that you intend to use the rifle for. As a target or varmint rifle it will be a bit of a pain, but you can get by if you use it as a big game rifle. If, for example, you have a duplex reticle, you can make use of the ends of the wide post sections as hold off points; the bottom of the top one will bring you close to zero at near range targets, hold center for moderate range, and use the top of the bottom post for long shots. Likewise, the left and right posts can be used to hold off for windage.
I suggest you pick a single load to use in your rifle, either factory or a handload, and work with it until you are able to fine tune the holdoffs at specific ranges. Now the chances are that the hold offs right on the post ends will not coincide with the range when measured in whole hundreds of yards (100, 200, 300 etc) but you will quickly learn where the hold needs to be relative to the post ends and the cross hair at specific ranges.
The first step though is to put up a 12" target and learn what that looks like through your scope. At what range and at what power does your cross hair and post end bracket that target. Congratulations, now you're using your scope as a range finder as well as a sighting aid. Now if luck is with you, perhaps if you shoot with your target bracketed between the cross hair and the top post, you will get a center hit on your 100 yard target. You might get a 300 center hit using the cross hair, so what hold do you need for a 200 yard shot or a 400 yard shot? Record the results, one of those write in the rain pocket sized notebooks is handy. The more you use this system, the easier it will become.