The problem with people not available for a raid night will not be fixed by reducing the number of raid nights. Removing 1 night doesnt make these raiders populate the other raid nights. The reason have been missing is because of vacations, sickness or unexpected or planner irl things that have more priority than wow. If we raid 3 nights, and some are sick/working late/on vacation/going to the movies or what ever, then we'll still be too few core raiders.
Now, i do not think we have too few players who can only raid 3 nights, we have very good core attendance on sundays. So atleast most people CAN raid 4 days, so this is more a question about if you WANT to raid 4 days. Personally i feel like there will be people against 3 days and some people against 4 days, so then i'd prefer 4 days since this gives more raid time for progress and something to do in the evenings.
Also, we are making changes to recruitment, we'll have a dedicated recruiter now focusing full time on spot recruitment. If we are missing people for raids because people stop playing wow, leave the guild, starting ro show up for raids irregularly and everything else that is bad for the raid team, then this person will bring new raid ready players into the guild.
And for those who are sick/working late/no elictricity and other unexpected things, we need reserves. For this we'll do a rotation most likely since we'll stop farming people from HC (it leans towards this looking at the other post about DKP). This means players will have some free time from wow because you HAVE TO stay out evey 4th week or so.