I cut from 14-15%bf to about 12% and would like to lose the last 4% slowly by gaining muscle and slowly losing a little fat in weekly cycles time to land me at 8%.
If I eat above maintenance on training days (4) and below maintenance on rest days (3) , could my body build muscle on surplus days and lose fat on the deficit days. I will be net positive 500 cals at the end of the week.
Your body doesn't work by the cycles of the Sun. It is constantly burning or storing fat, building up or breaking down muscle. If at the end of a significant enough time period you've spent time building up muscle and burning fat, you'll notice you'll ook bigger and leaner.
That's hard to accomplish but your approach is a common one. Whether your 50p calories surplus comes out as muscle gain or fat gain will be pretty dependent on your genetics (assuming you're not sitting on your Ass eating donuts ) but I think the approach you outlined gives you your best opportunity to maximise muscle gain.
This is the Holy Grail of natty BBing--a nutrition/workout regimen that allows one to lose BF and gain LBM simultaneously.
Is this possible? Individuals far more informed than me (eg, Lyle McDonald; Alan Aragorn) believe it is. However, the particulars of a regimen that will deliver these results are far removed from what you've described above. I would suggest you seek out what these (and other) experts have to say on the subject. Best of luck.
While it's not as clear cut as the body actually synthesizing new tissue on some days and prioritizing the oxidation of fattys acids on others, the approach of cycling nutrient intake based on the demands of the day is a sound one, and has indeed allowed intelligent trainers to add muscle while tightening up ("recomposition").
As long as you're covering your basic nutrient requirements on the days you're hoping to lose fat, it won't matter if you're somewhat under your caloric numbers a few days out of each week. Like someone else said, the body doesn't really work in little cycles like that. Yes it's possible that you might hamper the minute amount of growth that could theoretically be occuring on your "cutting" days, but remember that your body is in a constant state of growth and repair (whether it has the materials or not) as well as running off of both glycogen AND fat (in different ratios depending on what you're doing).
Your body stores carbs and fats, and while it doesn't really store protein in any form to reaccess later on (despite a circulating pool of AA's), I would think that so long as you cover your protein bases, get your minimum amount of healthy fats, and enough carbs to get a productive training session in, even on your low intake days, you'd be just fine,
I had a dexa scan when I was at my fattest and I was 15% @205 since then I dropped 10lb, 3 inches from my waist, and got bicep veins lol (I have thin veins so I would never see them at high bf) so I am pretty confident i am between 11-13%.