Hi all,I have finally found an independent shop to do my trans ,not easy in my country trust me.
Current situation is a low mileage trans that pops out of 4th 90% of the time on load,
really a PITA ,the other operation in any other gear is fine and no odd noises underway,oil was very clean when last changed.
My main question is for those with experience is what parts will I need to cover this repair ? as I need to plan ahead,TIA.

Hard to say for sure until the box is apart but I'd think you may need:

New 4th gear dog teeth
New 4th/5th shift sleeve that engages with the 4th gear dog teeth -- hard to say which is worn - this or the dog teeth - probably both.
Sychronizer band for 4th gear.

Potential items of concern:
Shift fork for 4th / 5th gear. Since this is shared and not popping out of 5th - probably OK but could be bent or biased toward 5th
Shift rod detent worn out? Again seems unlikely since 5th is OK.
Broken detent spring - as above seems unlikely since 5th OK

For sure you need:
Copy of Dr. Evil transmission repair DVD.

Synchro dog tooth removal tool P225

Shaft alignment tool P260

New intermediate plate gaskets -- These are select to fit and affect pinion depth. They come in 0.1 mm, 0.15mm and 0.2mm. You won't know what you need until you take yours out and measure the thickness. I'd order all three so you have the right one/combination.

New Output shaft seals (might as well . . .)
New Input shaft seal
O-ring seal for differential side cover - its a 50 year old rubber seal after all.
new gasket for trans end cover

Carquip in Colorado sells used trans parts.
If you get stuck, reach out to me and I will do my best to find what you need.

What you are describing could be a misaligned 4th to 5th gear shift rod, might be just a readjustment with new gaskets and good to go.

If there is problems with the 4th gear synchro, the previous rebuilder may have swapped 2nd gear synchro to extend the life. The dog teeth are pricey, synchro bands are a bit less.
I would recommend the Dr. Evil DVD also.

Before you dismantle the tranny, double and triple check the shifter adjustment.

Wow very kind of you,could take you up on that,cheers.
Phil I'am really thinking its a detent issue...worn or damaged/weak spring,it pops out of gear with no fuss ,wishful thinking but the trans only 50K miles on it and never been apart.
Shop has all the tools for 901 and 25yrs experience in all models so have no worries using them and besides fixing small to huge engines happy to say I've never done a gearbox ever so prefer to leave it with someone the skills and correct tools.