Do federal quarterly income tax withholdings have to be the same amount, or could I do (for example) $1000, $1000, $1000, $25,000?
Ahhh, estimated tax payments. If you want to make them and not be subject to a penalty, they generally need to all be the same amount. However, if your income varies greatly during the year, you can make unequal payments (reflecting, proportionally, your income variation during the year.) There is a whole separate form/worksheet to fill out to see if you can do this.
However, the whole underpayment penalty is something that scares people more than it should, IMHO. Southerdoc mentioned "penalties + interest," but that's not completely correct. The underpayment penalty is essentially interest on the difference between what you should've paid (on the date that it should've been paid) and what you actually paid. The penalty rate is 8% annually (for 3/4 estimated tax payment dates) and 7% annually for 1/4 payment dates. So, it's more like just interest. Therefore, I generally choose NOT to make estimated tax payments, gambling that I can earn more than that through investing that $$$ throughout the year.
This isn't really a gamble (for me, at least) for a reason that I will explain. First there is the Safe Harbor provision that was previously described. Also, it sounds like you are self-employed. However, if you also have some income from wages, the IRS counts all withholdings as if they are contributed equally throughout the year. Thus, during Dec, you can increase your withholding to pay the tax you were supposed to pay through estimated tax payments, and NO PENALTY.
But, if you are not 100% comfortable in your understanding of all of this, by all means, like others said, GET A CPA! (EDIT: Whoops, looks like you already do!)