A little about me - I have been breastfeeding for a year and and starting to wean - I have had many ups and downs with supply so I have plenty of experience here (had a stint where antibiotics reduced supply heavily and I had to work it back up).
Congrats on breastfeeding this long already! It is the greatest gift that you can give to your little one. This part can be really had work, but if you get over this hump you should have smooth sailing ahead. It is around this 3.5/4 mth mark that your body really starts to listen to your little one. Breast feeding is all about supply and demand. When you supplement with formula it is a downward slippery sloap because you are sending a clear message to your body that it is making enough and that it does not need to do more. You continue to do this - your body continues to make less.
You need to undo these messages that you have been sending to your body. Here are a few suggestions.
1. Your baby is your best trigger to make your body produce more milk. Any time I have had supply issues I have just nursed for longer. Your body produces milk as your little one feeds so, if you feed long enough (and if you have a patient little one), the supply will come (just keep switching sides every few minutes). During downtimes of supply, I sometimes fed for an hour at a time. I have never supplimented with formula and never supplimented with my stash if I were present to feed.
2. Insert more feeding sesssions. If you are feeding every 4 hours, try every 2 or 3 hours for a few days until supply improves.
3. Any time you suppliment with formula always pump immediately for at least 10 minutes on each side. You might not get anything and that is okay. You are not pumping to get a stash - you are pumping to artificially send a message to your body that it is not making enough and that it needs to produce more.
4. I have heard that the combination of fenugeek and blessed thistle (both herbs that can be found at any health food store) can increase supply. These herbs are also found in "Mothers Milk" tea that can be found sometimes at the grocery store. They don't work when used alone. Also, I have heard that they will cause your little one a bit of gas.
Are you still feeding at night? If not, insert a pumping session at night.
In general, following any of these recommendations, it will take about three days before you start seeing and increase and 1 to 2 weeks before supply is back to normal.
Couple of other things - remember that your little one is the best thing for your supply so feeding longer and more often are the best suggestions. Also, drink lots of water and get lots of sleep. There are also perscription drugs you can take to up supply but I don't think that you need that.
I am so sorry that your husband and mother are not giving you more support. I encountered a bit of this (especially after the 6mth mark). Unfortunately in our parent generation there was a huge push for women to stop breastfeeding with the women's lib movement. Thankfully bfing is making a comeback in great part because so many (even formula companies) understand how much better it is for babies. Here is a list of 101 reasons to breastfeed. If either your mother your your husband has not seen this before, maybe you could show it to them and maybe it could help them understand http://www.promom.org/101/
Also, it is nearly impossible to do this without support. The hopital I had my son at (Trident hospital in Summerville) has a breastfeeding support group once a week with a free lactation consultant. It was such a HUGE help for me when I had any questions. If there is such a group anywhere around you, I would highly recommend going.
Sorry this post was so long! Good luck!