Partial boil question

I have a quick question on partial boiling. What is the disadvantage to doing a partial boil? I do not have the tools/capacity to do a full boil. I think I can get away with a 2 to 2.5 gallon boil without an issue.

I will be doing 4 gallon batches as the buckets I have acquired are 5 gallon buckets, if filled to the brim. It's my understanding that I need to leave at least some kind of head room for the fermentation process so I do not get a crazy blow off.

So would it be logical to boil between 2 and 2.5 gallons, then top off with 2 gallons of water? Would it be diluting way to much? Can you dilute way to much?

If this can be done. What changes would I have to make to a recipe in order to make this work?

I have tried to do a forum search but I can not seem to get a straight answer for the situation I am in. Forgive me for being such a newbie, but I finally got a gift card to the local brew store and want to start with an extract recipe, but not with a kit.

2.5 gallons is plenty of water for an extract batch with steeping grains. Even topping up to 5 gallons.

Plenty of great beers made that way. That was where myself and probably a lot of us started.

Just need to find a recipe and have it scaled to 4 gallons most online kits and recipes are for 5 or more gallons. You can roughly do 4/5 of all extract grains and hops to scale it down from a 5 gallon recipe

I figured the scaling would not be to hard, but would I add the whole addition of the DME/LME and hops while boiling the 2 gallons?

I plan on following a recipe my local beer store has. Are there any tools that can do the auto scaling from 5 gallons to 4 gallons and boiling amounts?

Try beersmith2 this is what I use and it works pretty well I think they offer a trial so you can test it out. It is well worth the money IMO

For extract brews I did half of extra at beginning and the rest at flame out. And if using dme and lme use dme beginning and lme at end. Beersmith2 even accounts for late addition

Hops just follow the recipes schedule