35 secs at f8 is way more than I get. As I said I even had to reduce the wattage to 75W because my Paterson colour analyser had a min exposure of about 4 secs and I couldn't null the exposure dial so needed less than 4 secs.
Unless there is something wrong with the aperture movement on your f4, don't bother to buy a f2.8. f2.8 isn't going to improve the light dramatically. Have you checked that the lens' aperture setting is operating properly. When you remove it from the enlarger and hold it up to the light you should see the aperture open and close as you move the aperture setting.
When the seller inspected it on its return and pronounced it OK, did you insist on seeing it in operation? That way the seller's OK and your not OK might be the same thing but 35 secs at f8 for a 6x4 print strongly suggests that the seller's OK is not OK.
Can you find anyone nearby from a camera club who still has some knowledge of enlargers? Unless you can get a knowledgeable second opinion I fear that we never get to the bottom of this collectively. 5 mins in your darkroom with another user might solve this but short of that I am beginning to despair of getting to the bottom of this.