Hi BumbleNova,
No comment on the house purchase or the tax issue - plenty of sensible things have been said already! However, I think it is worth considering a different point of view when it comes to him asking you to marry him and getting defensive when you ask him about it...
I was in a similar position a few years ago - DP and I had been together for more than 7 years (since our early twenties), had already had lots of conversations about the future, and had already bought a flat together. However, there was no sign of a ring - I was beginning to wonder if we would ever get married!
Like you, I tried to bring it up jokingly while we were on holiday, and, like your DP, my DP was not too pleased about having that conversation. But it turned out that it WASN'T because he didn't want to marry me - in fact, he proposed less than six months later (and, it turns out, was already looking at rings when we had that awkward conversation).
When we chatted about it later, he said that he got defensive because he sort of felt that asking me to marry him was his decision, and that he wanted it to be something that came from him - not something that looked as if he had been talked into. He was worried that the fact that I had brought it up meant that, when he did actually propose, I might think that it was just because I had asked him to, and not because he genuinely wanted it. We are now very happily married with a baby on the way.
Anyway, the point is that you shouldn't assume that he doesn't want to marry you just because he hasn't proposed yet. In today's world, four years is not a ridiculously long relationship - he may be completely committed, but just not feel quite ready for marriage (he may think of it as something that "grown-ups" do, and might not think of himself in that way yet, for example). If he really does not want to marry you, then pushing the issue and giving ultimatums will not make him any more likely to want to do so. And if he does want to marry you, then giving him an ultimatum might sour the whole process.
Ultimately, you need to ask yourself (and only you can know) whether you believe that his commitment to you is genuine - with or without the symbol of marriage. I knew that DP (now DH) was fully committed to our relationship - although I wanted to get married, I didn't think that not being married meant that he was not committed to our future together. Take the house purchase, the tax issues, and the marriage out of the equation - is this someone who you are committed to spending the rest of your life with, and does he feel the same about you?