I followed the walkthrough available here but Otome didn't reach stage 4 when she was supposed to (i probably picked a wrong choice somewhere along the way) and now I have no more events with her on Friday. Will this have any negative effects and, as a last resort, is there a cheat code that will let me change her fate?
Trespassers will be shot. Survivors will be shot again.
Unless you're going for Triumphant her not reaching Fate 4 doesn't matter (except that she won't join you on Saturday, unless you're on the Law route, and that even if she does she may not reach Fate 5 so you'll miss out on that demon ). If you are going for Triumphant then you can't get it without here being Fate 4 by the end of Friday, so you're out of luck.
The walkthrough is pretty much completely correct (I crosschecked the alphawiki, the translation of the alphawiki on this board and the FAQ, and the game as I was playing, and the only thing I noticed missing was the +2 Fumi for persuading her before you attack her on Monday) and everyone leveled up when stated (I think you have to skip the Billiken fight and do the other option at that time too but that's not related to Otome) for the Triumphant entry with AO at 4 (thus letting you get into the Kignmaker route as well) , so yeah you must have missed something or answered something incorrectly.