I don't think they'd lose fans. It'd just delay the already speeding hype train. There aren't going to 'lose' money if they don't announce Fallout 4 just yet. We know that a Fallout 4 trailer has been commissioned or finished so it'll come at some point... we just don't know when. Fingers crossed for E3 though!
I'm already riding that hype train: link
This may not mean anything as it comes up as generic bethesda title, but if you follow the pre-order link to amazon, it shows a release date of December 31, 2015 (xbone). [http://www.amazon.com/dp/B00YQ1NOPM]
Found this PC link at best buy but shows no release date...[http://www.bestbuy.com/site/fallout-4-windows/6790101.p?id=1219248891405&skuId=6790101]
Clicking USA -> Amazon -> Click the Preorder icon, takes me to http://www.amazon.com/dp/B00YQ1NOPM, which says "This item will be released on December 31, 2015."
Other options I tried just gave previous Fallout games for sale (I tried UK->Game , UK-> Amazon, USA->Steam), no Fallout 4 specific item