So I added a gallon of tap water prior to fermentation...

So I brewed my first batch last week and when I put it into my primary I only had 4 gallons, without thinking I threw in a gallon of tap water. So my question is will this ruin my beer since I did not boil the water?

Probably not, might have some off flavors, but than again you might not notice

Plenty of people top off their fermenters with tap water. It'll be fine.

I agree do not worry. I do it all the time with kits

I did it yesterday too, and just about every time I need to top off. I never boil the water.

Is there a boil water advisory in your community? Did you take a drink out of the tap, or wash your dishes, or brush your teeth using it....Do you feel the need to boil water for those things?

When I was doing extracts as partial boils I would always boil up a couple of extra gallons the day before and chill it in a fridge overnight. Helped with the cooling this way as well.

Should be fine as long as it does not have a lot of chlorine in it. I have done that as well but I used bottled water.

We get water through another city on the shores of Lake Erie. Soft water with no chloromines that I ever detected. works pretty well for many different ales. So a little chlorine is ok,since the amounts are in "Parts Per Million",or PPM. not enough under normal "nature" conditions to worry much about. Least of all 1 gallon.
I had started early on buying spring water from Giant Eagle (from Ohio springs) for 79c a gallon. Then I remembered the 2 local spring water sources for 10c a gallon! Works even better than the store stuff. I buy 6G for a 5G batch,putting 3G in the fridge a day or two before brewday. After the wort chill & strain into fermenter,the cold water gets the wort down into the 60's. Low 50's if I'd let the wort chill go too long. but it works great for geting down into good ale yeast ferment temp ranges.

This is what I do for my brew days (or the night before if the brew day starts early)

Guesstimate how much water you'll need. If yuo have a 5 gallon extract batch you'll need at least 6 gallons of water maybe more depending on how strong your boil is. So what I do is take any container that won't be used for brewing (another kettle, growlers, pitchers etc.) and fill them up with that much water as you need, or if you come a little short can. I filter all mine using just a plain old brita filter. WOn't get rid of all of the contaminants but enough for my liking. After you've filled up your kettle and started a boil/mash you have an hour of down time, so then you just go and filter off some more water and keep some reserved. It's water, it's like 2 tenths of a cent per gallon, and you can drink it, so if you have too much prepared, drink it later.

I saved 1 gallon spring water jugs from the store to get my local spring water with. I starsan them & drain well right before leaving to go get it.