Altitude does not work linearly. An extra thousand feet at 18k is going to feel very different than the same at 12k, so you should play it safe and take the time to acclimatize properly. It will make your trip so much more enjoyable.
For a 4 days trek, Santa Cruz is a no-brainer, and a really beautiful trek, though it can get a bit crowded. You can also do day treks from Huaraz (laguna 69, laguna Churup among others), of varying difficulty, so taking two or three in a row can help with shorter acclimatization schedules.
I have had a really good experience with the agency "Andean Kingdom", just opposite to the Casa de Guias, both for organizing treks and climbs. And of course Ted from Skyline Adventures is a great guy and I haven't heard anything but praise of his services.