It isn't crazy but is unwise I think to do so without letting at least 6 months or so go by so that you are not doing something of such huge long term commitment straight out of the pain of loss-- because what you are missing is Abbey, not simply missing a dog in the house, as there are three other loving dogs already there that need you right now. Four is a lot of commitment -- and any time anyone considers this breed they do need to think whether they are able for the terrible journey you have just been through -- how would you cope if you had three months of a new cavalier to find it also has SM? To face that challenge again, you need to give yourself plenty of time to heal. It would be a hard burden to go through again but especially so soon.
I think it is very important to take time to grieve and give the lost companion plenty of time to move from fresh and painful memory to a gentler place in the mind. You have three dogs that need your care and love and who will also be missing a companion -- and have probably not had as much time understandably given all your worries with Abbey (I hardly know anything about any of your dogs except Abbey -- I think all of us would like to get to know them better over time now
).
It is also hard on both people and the new dog to bring in another dog as a replacement or to fill a gap when it will be a completely different personality and individual in its own right. The existing dogs have only just begun to adjust to one longtime companion missing -- which is stressful for dogs too -- and adding in another is another stressful situation.
I think it does take months, sometimes a year or more, to properly absorb the death of a loved companion and to be fully ready for another * in its own right* -- meaning not just as a replacement or a new entertainment or something to take the mind off loss (in the same way that we need time to grieve human friends and family when they pass away).
I do think, from long experience, that it does not benefit either new dog or human to rush to get a new dog when one has died. It is a terribly distressing and complex emotional loss to lose a dog, especially one that has needed extra care over time. Be sure to take the time to honour the lost one and be in a positive place before thinking about another. I'd wait at least 6 months before even considering another so that everyone has more perspective and time, and so that hen you are ready, you can begin to put in the time and effort to locate a good breeder or to work with a rescue.
