4" bunks are too narrow in my view (and that view is shared by many here). I've widened mine already and my only regret it that I didn't make them a couple inches wider.
On my trailer, the bunks were easy to modify because they consisted of a flat board, covered in carpet and screwed to a metal U-channel. All I needed to do was swap the board against something wider. Some trailers seem to have something like a 4x4 held up by two brackets. To change that into a wider support might mean adding a flat board on top. The DS hull really doesn't like to be supported in a narrow area.
The 60" sounds good, definitely better than 48". if the flat area of the hull is longer, you can let any added wider board also overhang a bit front and or aft. (I added a substantial overhang aft - hey, any increase in bunk area reduce the point loads).
On the question 40" vs. 59" I gave you you my best answer already. Ideally, your bunks need to be directly under the place where the seats connect to the hull (if you have a DS1 with molded seats tabbed to the hull). The lineup won't be perfect as the seats angle in a bit towards the back. I believe at the CB they would be about 32" inches apart (but you should verify that, I don't even have my boat close at hand as I write this). That would seem to indicate that with a bit of a wider bunk than your 4" a spacing of 40" would get you close. Also, mine tilt in to meet the angle of the hull, making the distance between centerlines of the bunks a bit less than the distance between supports attached to the trailer.
Mine are set up to within the limits my old trailer supports, but I can't give you that in inches. However, I was looking for something else on my PC and stumbled on this photo:
which shows my trailer positioned for retrieval. The trailer is a touch narrower than the 6'3" beam of the DS, so you might get some "measurements" off that picture
.
You can see how I drop just the aft roller down to water level. I found that if I force the bow to roll on, it keeps the boat aligned until the bunks start lifting the back (the front is steeper and sits between the bunks on the rollers). Aft roller is dropped an inch or two so that the trailer can sit that much higher (and the bunks will engage a bit earlier) and also so that there's no load on it while driving. Took me a while, and many botched retrievals to figure out that this was the way to go.
After these tweaks, I can positively state that additional guides would add nothing to may setup. Any guides work best, I think, if your retrieval mode is set to "float on", not to "roll on", something I use with a different boat / trailer combination.
(Oh and nobody ever needs to get in the water).
Good luck with getting yours set up to satisfaction!