Help: Don't want to regret this

Well, it's a little difficult as you didn't note the model details however . . .
As all 4 bikes are in a similar price range then they're likely to have a comparable spec level.
You should arrange a test ride on all 4 bikes and see which you feel the most comfortable on . . . did you say you'd been measured and were looking for a 61cm frame - depending on leg measurement I guess that makes you around 6'2" to 6' 4" right???
I'm 6' with an inside leg measurement of 32cm and I ride 56cm and 54cm road bikes but feel more confortable on the 54cm.
Is a $800 bike twice as good as a $400 bike? Well, no, not exactly, there is a law of diminishing returns in these matters . . . try justifying spending $6k+ over your $800 budget. Or perhaps try and prise the $75k 7-Diamond bike from the lucky bidder at the LAF auction.
Would you be happy on a $400 bike if you hadn't tried the $800 bikes? Probably.
Would a $800 bike give you years of riding pleasure as you make your way through recreational cycling to pehaps racing? Well, it would be a great start but then your performance is down to you to a very large degree . . . Graeme Obree broke the World Hour record in '93 & '95 on a bike he built himself using washing machine bearings.
Buy the bike that fits you and you feel comfortable on and then get out there and ride the thing.

As for your supplemental questions . . .
1 - How many gears do you chosen bikes have? Are any of them compacts? What spread does the rear cassette have?
If it's a standard double then it's likely to have 53/39 rings on the front and something like 12/25 on the rear cassette. Whether you have a 8, 9, or 10 speed cassette on the rear that is likely to be the spread.
Yes, more gears give you more flexibility within the range to be in exactly the right gear for your fitness/strength/capability but is it a show stopper??? Nope, get out there and build your strength.
2 - I can't comment on the bikes as don't know the models that you have seen.
3 - It's up to the bike shop if they are negotiable. They have overheads to cover after all.
Can you get all the expert advice from the, hopefully, lovely people at your LBS and then go and buy the same bike a couple of hundred bucks cheaper from the internet "stack 'em high, sell 'em cheap" sites? Yes, of course you can . . . but you might need your LBS's expert help from time to time and let's be honest, if everyone took their advice and then bought from the internet then you very quickly won't have your lovely LBS with their knowledgeable and helpful staff as they'll have closed through lack of business.
I was in a great shop this morning and the guy looking to buy a bike was getting great advice, he'd done his homework and knew that he could get it cheaper on the net. The salesman said he couldn't match the price but they do throw in a number of free services as part of the deal that actually made them cheaper in the long run so make sure you know what the whole deal package is because if you do end up getting into racing you will find your LBS to be an invaluable resource.

Within a given price range you're not likely to see much difference in the quality of the components or the overall bike at least for new bikes that aren't on sale. There's too much competition in the bike industry for one brand to be a much better deal and unless they're appealing to a very high end audience there's not much room for one brand to charge a lot more for the same frame type and components.

One possible exception is Giant only because they're one of the largest if not the largest manufacturer of bicycles in the world and many bikes that carry other labels on the showroom floor are actually built in a Giant factory (All but the highest priced Treks for instance are made by Giant). But still in the price range you mention there's not going to be much if any quality difference based on brand.

The more important things are fit and subjective feel of the different bikes. There are differences in geometries or different saddles, stems, etc. that can make bikes feel very different from one another. Sure you can swap most of those components as long as the geometry, fit and handling work for your riding style but I'm guessing at that price point you want a bike that feels good to you without changing parts around. So go back to the shop in comfortable clothes and start test riding different bikes to see what feels good to you and to find out what fits you. That's far more important than the exact components.

Have cyclists ever felt the need to upgrade after buying what they hoped would be their last bicycle purchase? You bet! Have they 'needed' the upgrade? More often than not, no! But if you ride a lot and really love being on a bike you learn to appreciate nice bikes and it's pretty natural to upgrade either because your riding evolves or just because you like riding a really nice bike. If you've got cash to burn then buy the nicest bike you can afford right now. But otherwise don't sweat it, buy what you can afford, get out and start riding it and if you end up upgrading down the road it will be because you've gotten hooked like the rest of us. Welcome to the club.

Good luck,
-Dave