I would be avoiding the long stretch between feeds during the day mostly because if for some reason we had a long break between feeds we ended up cluster feeding all night to make up for it and not getting any sleep. If she is having a weight gain issue then it will also help to wake and offer feeds too after 3 hours.

I really don't think one low week is a worry IYKWIM and there is a good chance she will have a big weight gain the following week. With all of mine there has been a lull with weight gain at 3-4 weeks and then it takes off again. If she is having constant weight gains of only 30 grams then it may be time to worry but try not to be concerned just yet 90g and 100g are pretty good and not far under the "ideal average" which if I remember correctly is 125g. I know it's hard at the time but remember if she is having lots of wet and dirty nappies then she is getting enough.