I think 100ml is good for a 4 week old (corrected age). Is he having about 6 feeds a day?
It is not uncommon for prems to take longer to feed and for it to tire out quickly.
Have you tried the next teat size up so he doesn't have to work so hard...if he splutters on it go back tot he smaller one.
Is he putting on weight? Plenty of wet nappies?
If so then he is probably getting enough.
If you are concern it might be worth talking to you CHN.