I know how incredibly frustrating it can be to try to get baby off the shield. But please don't feel you are 'stuck with it.' If I am understanding correctly, your baby was able to nurse for four weeks without a shield. He will be able to again. It takes persistence and it takes patience, but even when they are introduced much earlier, it is possible to wean off nipple shields in almost every case. Also, while I hate to say it, I am concerned about the advice you are reporting your were given. A lactation consultant who gives a mother a nipple shield at four weeks due to fast flow, and no other reason, and without trying any other methods for reducing flow or helping baby handle the flow first, is unusual. Every lactation consultant is aware that nipple shields are a very controversial piece of breastfeeding equipment. This does not mean they do not have their place, they do. But it is not something a lactation consultant who is up on current thinking about their use (and misuse) is going to give to a mother as anything other than basically a last resort. When my IBCLC suggested it to me, my baby was a week old, jaundiced, not gaining, and took an hour to get latched if he was able to latch at all, and breastfeeding was going to have to end before it even got started if we did not find a quick solution to his inability to latch and nurse. And still she pulled it out of her bag like it was radioactive and explained to me all the possible drawbacks in detail.
Did the latch pain start after the introduction of the shield, or was it always painful to nurse? Was gain always normal before and after the shield? Was tongue and lip tie discussed, any different latch techniques explored? I am still wondering if there is more going on aside from fast flow. I wonder if there is some other reason baby has a hard time nursing and that is why baby is resisting weaning off the shield. I think it might be worthwhile to see another LC- a board certified LC (IBCLC) if possible.
Here are some links to more latch info and weaning off the shield info:
Nipple shields: http://kellymom.com/bf/concerns/child/wean-shield/
nice overview of different latch/positioning ideas: http://feedthebabyllc.com/latch-and-positioning/
in depth explanation of nipple sandwich technique: http://www.llli.org/llleaderweb/lv/lvfebmar04p3.html
Description/recommendation of what is involved in a typical lactation consultation: http://cwgenna.com/lconsult.html