I am a very strong supporter of nursing, and extended nursing into the toddler years. Now that I've said that, I want to point out that it's not an all or nothing situation. You can do a combination of formula and nursing, and that will be better for your baby than only formula. If you nurse exclusively for the first x weeks, and pump, and introduce formula, and do a combination, it might free you from feeling like the nursing is a burden so that you can enjoy it, and all the wonderful things that nursing can bring to your relationship with your baby.
Milk pumped at 4 months is okay for an older baby. It's not ideal, but it's better than formula, and it definitely will not hurt your baby at all. According to the La Leche League guidelines (found at http://www.llli.org/FAQ/milkstorage.html), if you make sure that your hands and equipment are clean, breast milk can be stored:
* at room temperature (66-78°F, 19-26°C) for 4 hours (ideal), up to 6 hours (acceptable) (Some sources use 8 hours)
* in a refrigerator (<39°F, <4°C) for 72 hours (ideal); up to 8 days (acceptable)
* in a freezer (-0.4 to -4°F, -18 to -20°C) for 6 months (ideal) up to 12 months (acceptable)
Another thing that I suggest, even if you don't go the combined BF/Formula route, is to have 1 can of formula in the house for emergencies -- like the time you took a break and your husband spilled the milk -- just in case. It's tricky -- if you've decided that you're not supplementing, having the can in the house can be tempting if it gets difficult, of your tired, or something, and it's sitting there all nice and easy to use, but it's possible to resist if you want to.
Again, supplementing with formula does not make you a bad mom. If it allows you to relax about nursing and enjoy it and be a better mom, then it could be the right choice for you.
I completely understand that tied down feeling -- I've nursed 3 children for 36 mo, 39mo, and 23 mo (so far-- he's still going strong) and there have been times, particularly recently when I think about the fact that I've been continuously pregnant, nursing, or both for 10 years, when I'm really ready for my body to be just mine again, and nursing has been an extremely positive experience for me. I can only imagine how hard it must be if it hasn't been so positive. Good luck.