As I understand it, you have a 401(k), and a one-time opportunity to convert it to a fixed income life annuity provided by your employer. The essentials here are not that the annuity "would essentially double what I would receive versus withdrawing 4%/yr. if I continue investing like I am." Be careful. That proposition is simply the general life-annuity-versus-portfolio-withdrawals thing. Be aware that the traditional 4% is neither "4% of whatever the portfolio value is" nor is it "4% of the initial portfolio value," it is "4% of the initial portfolio the first year, and adjusted upward for cost-of-living in subsequent years." So, good, bad, safe, or unsafe, the 4% withdrawal rate assumes inflation adjustments.
Anyway. Don't compare the annuity to "4% withdrawals" to make your decision, at least not yet.
The big thing here is that you can buy an annuity from an insurer any time you like. So one of the things you need to find out is how this one-time opportunity compares with what you can just buy for yourself. Two places where you can get actual dollar quotations online are BRK DIrect EZ-Quote, which quotes on flat, level-payment annuities (same number of dollars every month), which I think is comparable to what your employer is offering. I'm in a hurry and can't post the link but Googling should do it. The other place is Vanguard, but you must have an account. Drill down to "what we provide" and "fixed annuities," and you will then need to, essentially, open a second (free) account from within your Vanguard account at a place called "Income Solutions," where you can get a large and useful variety of quotations, including quotations on inflation-adjusted annuities.
Flat, level, payments are not really what you'd like to have, and 50% to survivor seems stingy to me.
Even if the annuity turns out to be a good deal compared to the competition, you may well not want to do it, it all depends. But the first step is to find out whether it's meh, good, or fantastic.
The other alternative to be considered is NOT starting Social Security, but intentionally living on and spending down your savings for a few years in order to delay Social Security and get a bigger benefit. This is logically very similar to buying an annuity, but is typically a better deal.