F4 being just one stop slower than F2.8, it would indeed be interesting to try taking a star photo just to see how much of a difference there will be. With film there was something called reciprocity failure which meant that above and below the film tolerance, exposure could be outside expected limits. Thus, a 45 minute exposure might well produce results under the same conditions as a 5 minute exposure or even as a 90 minute exposure. It would not be unreasonable to expect that digital sensors exhibit a certain amount of reciprocity failure and a 30 second exposure would place film at the lower end of reciprocity failure.
So, the proof of the pudding being the eating, I intend to have a go at star photos with an F4 lens. Anybody that knows me in real life knows that I'm very willing to challenge asserted facts. As an example, I'm quite happy to compete against .22LR rifle shooters on a 75 yard range with a .177 air rifle. Given the right pellet (which I have) my accuracy is just as good yet the commonly held belief is that air rifles are good only to 25 yards. The commonly stated belief is that you can only do star photos with a minimum of 3200 ISO, a 30 second exposure and an f2.8 lens. Well, baloney to that! I did mine with 1600 ISO, a 30 second exposure and an f2.8 lens and they were quite decent. Changing more of those variables could be entertaining.
If there's one thing I have learned in life, it is that people are all willing to peddle bullshit and to believe that bullshit. If ever there was a call for an anti-bullshit campaign, it should be very loud when "facts" are stated definitively. Heavens above, outside of photography I have people telling me bullshit. This ranges from insurance salesmen to civil servants and more. One entertaining piece of baloney was given to me the other day. I'm quite happily working where I am and somebody was trying to tell me there were wonderful jobs to be had in another organization and that I'd have to apply as a warehouse operative and make a sideways move. Uhm.... Organizations don't work like that. They class a warehouse operative as manual labor and manual labor stays as manual labor forever.
Photography, just like life in general is full of bullshit kings, queens and divas. Go to any photography forum and they're all parading their baloney. In years gone by, they would have paraded to a more limited audience in clubs and magazines. Now they can parade online in forums. It should not surprise regular readers that is why I barely frequent such online dens of iniquity.