Yes, it's normal. However, 4 hourly feeds is unusual, as is sleeping so long and TBH that would ring alarm bells with me because I would be concerned that they are possibly unable to get enough milk in those times. I say "possibly" because some babies can work within that sort of routine just fine, but it's more likely with this situation that the baby (or babies) are not "demanding" or requesting sufficient milk, and therefore mum's supply drops and then you end up in a situation where they are now. Nipple shields can also lead to a lowered supply (although they are brilliant in many circumstances and I'm certainly not saying you've done anything wrong by using them - they just need to be used under proper lactation support).
What they are probably doing right now is trying to build up your milk supply and this is likely to be possible, but, the question is - what do you want to do? If moving to formula is right for you then there's no point going over the "what ifs" and what's happened, and I felt from you post before that this is where you're at. In that case you'll get lots of support from here if you need it. If you are still of a mind to continue then you'll get plenty of help here, as well, to do so.
Sorry - I'm just not sure what YOU want, but whatever it is you'll be looked after here :) (If you want to be, that is :) )