Marko, I'm assuming you want to stay with Nikon. In that case I think you should look at the F100, if you want to replace many of the features of your F5. Or, look into an F3 and pick up a flash bracket for it to give you that old style press photographer look.
And I agree with Jim, in this case spending less is more.
I would go with the F3, but my flash is made for AF Nikon cameras. I'm thinking I'll buy a Leica III and a N90s.
A bunch of Nikons; Feds, Zorkis and a Kiev; Pentax 67-II (inherited from my deceased father-in-law); Bronica SQ-A; and a nice Shen Hao 4x5 field camera with 3 decent lenses that needs to be taken outside more. Oh, and as of mid-2012, one of those bodies we don't talk about here.
Best thing about the F4 is that its the Nikon "rosetta stone" in terms of bridging the gaps between all the Nikkor lenses and some other accessories.
If you aren't looking to preserve your ability shoot lenses from the 60s along with the current AF-S types and you liked the F5, another F5 or an Exc F100 makes more sense.
I would go along with the F100 as I think it's the best value in cameras right now except for one thing: if you killed an F5, the F100 doesn't have the rear seal that the F5 has. Yes, your SB-600 will work on the F4. I have both an F100 and a F4 and I use the F4 more; it just feels good - all controls are in the right place for me. You should try out as many of these cameras to find the one you like the best, buy it, and use it! Maybe invest in some rain gear next time out in the elements; of course, it is a good excuse to buy another camera-------
Have you looked into shooting medium format? Once I discovered how nice it is to have a larger negative, I haven't touched a 35mm camera. The F100 was my last 35mm camera and it was wonderful.
I considered MF for a while, but the camera size and features don't really suit my style of photography. In addition, the camera and films are a lot more expensive, and I don't normally enlarge past 11x14.