Welcome to my couples workshop, where people 18-80 of diverse cultures & orientations have come to explore their questions and find a path of heart. I'm about to turn 70 myself, and I've dealt with many people who are recovering from the loss of their lifetime spouse. Men are much more likely to successfully adjust to having a new mate than women are, and 4 years is a healthy period for grieving. I think you're actually lucky that he is so aware of his past connection, yet he also wants to make his relationship with you work out. 10 months together means that this is the first anniversary of her passing that he's dealt with since you got together.
I suggest a way you can honor his devotion to his own past and also help him achieve greater completion: Tell him that a therapist you met told you that everybody who was truly devoted to the spouse that has passed away needs to negotiate the permission of that departed (wife) to be able to commit themselves more fully to a new relationship. So he could write a letter to his former wife in which he explains his feelings for you, and how he feels about focusing his new love onto you instead of her, and then asks her for permission to let her go and give his living heart to you.
That experience of writing would give him a major grief episode, but it might only last for a couple of hours if he works his way through writing the whole letter in which he declares his intention to love you in this life. And he's free to make up his own mind about what he wants to do after he dies, since you also have that freedom. Perhaps you have made your own intentions known to yourself, vis-a-vis your own loyalty to your departed husband. If not, perhaps you could do the same in a letter of your own to your own long-departed husband. In this way both of you could Independently clarify your internal intentions towards your prior mates so that your devotion to each other would be more clear-cut in your minds, and your unconscious minds could further adjust to your new love. In fact, if you wrote your own letter to your former husband about your feelings about your new man and your past loyalty to him, even though it would be a lot easier than his writing would be, you could have an easier time helping your new companion with his transition. If your new companion wants to understand better what I'm writing, he could ask for me on this web site, he could write privately about his concerns, and I would advise him very specifically about how to proceed--and that would be a lot less expensive for him than seeing a grief therapist for one session. (I'd guess also, that a Hospice counselor or a pastor might help him as much or more than I can without the ability to see him face to face.)
It's also quite valuable to pay attention to your dreams, especially the night after you have written a letter to your departed partner as I'm suggesting. For dreams frequently come to help us find our way through grief to the other side and a newer way of living in relation to our departed loved ones. So you could also tell your companion that the therapist you've met could help him interpret a dream that is about his former wife or about you. Just be sure to ask for me by name, Norman Brown.
Furthermore, this is his first anniversary in which he must relate in his mind and heart to both you and his former wife. So if he gets the chance I've suggested to speak (via writing) clearly, honestly and emotionally inside himself with her, and perhaps, if needed also (inside of himself) with you, then his mind&heart will make a key step forward toward fulfilling his intention to give his earthly loyalty to you--as you have probably had an easier time giving yours to him. Just to be clear also: NONE OF THESE LETTERS I'm advising you or him to write are to be shown to anyone. They should be printed and then reread and burned or buried or stowed away in a private place, perhaps with a picture of the departed spouse that will also never be looked at again. Our unconscious minds respond best to rituals like this that are then made to disappear so that the thoughts and feelings won't be retriggered unnecessarily by physical remembrances.
Just as dreams are special symbolic communications from our unconscious minds to our conscious minds--which is how grief moments come up for us--a letter is an intentional communication from our conscious minds through our unconscious minds to the image of our departed loved one. So this letter should "disappear" and thus become unconscious itself, after it has been composed, thought, felt and read.
So by making his feelings and intentions explicit in writing and then making them disappear, he is also establishing a habit of both honoring her image inside of himself AND separating himself from it With her approval. I believe he WILL get her approval, for I have dealt with many dreams of widows & widowers about this loyalty issue with their departed spouses. I've found that the higher power (aka God for churchgoers) that communicates with mourners through their dreams WILL show them a spouse who supports their finding renewed happiness in their earthly life, if that is best for them and their earthly relations. In every counseling encounter I have had, Loving a new partner in earthly life IS BEST for both new partners IF they both want to love again. I don't want to push my own beliefs onto you, but my experience of 40 years acting as a professional counselor and dream interpreter has convinced me that LOVING is enabling Higher Power (aka God) to act through you. So it makes heart-sense that God would want us to love more while we are alive.
If you have any other questions about dealing with your partner's emotional anniversary of mourning, please ask and I'll respond. And please forgive me if my statements of belief have offended you. I'm not trying to convert you, but just to explain what my experience in counseling has taught me.
Thank you for this opportunity to express my understanding in support of your new love and your partner's further emergence from his widowerhood.
Norman Brown, Ph.D. LMFT