I specialize in the 800 but run 400 and 1500 as second events. So we know you are very strong from marathoning. What I do today that I did not do during my open years is run a lot of small hills and inclines even on my easy days. That strength helps me use my natural speed.
If you are looking for 4:30 then I don't think you have to have sub-60 second 400 meters speed. I've seen 8:34 for 3000 meters and 2:21 for the marathon from a runner who never broke 60 in the 400m.
I keep my schedule simple and on a two week cycle and rest anytime I want to rest. Breaks are taken easily off of 2 week cycles. Basically I run Sunday to Saturday with a long run, easy run, long repeats 6 X 1000 with 200 on small hills, rest day, repeat 16 X 200 with 200 on small hills, easy run, race Saturday as long as 5000 and as short as 400.
The next week the same except Tuesday is 16 X 400s with 200s and 16 X 100s with 300s. If I need a break from the small hills especially with speed work I run on a track. Oh, and I tend to train slower than race pace on the 1000s and 400s. On the 200s and 100s I run them mostly just naturally by feel. I don't want to press them. At the most I will push harder on every 4th one. And the Saturday races are not all out races every time but part of training. I go for it when I want.
I think running 200s and 100s up hills and jogging back down is mostly what I wanted to emphasize. Since you are already distance strong maybe you could rotate one week running them up hill and the next week on a flat course like the track. And what I find for me is that I'm really ready for a fast 1500/Mile when I start racing 800 meters about equal to or faster than my 200 repeats pace. Also before I am ready to really go for a fast 800 I usually run Saturday races in a pattern like this: 800, 1500, 400, 800. It works over 90% of the time for me. So you may try gearing up in that way of equal distance, longer, shorter then the Mile. So I do think it is good to do more racing than you are probably used to doing to adapt to it.
You can do it! I ran my fastest road 5K at 41 beating my 23 year old PB and I know a lot of sub-elite runners who ran faster in their 40s than they did in high school.
Sorry if my English is not good for you to understand.
Good Luck!