I have found this ballast on eBay which could either use a 35w SOX or 55w SOX bulb with a igniter built in. I have understood everything on this ballast but just there is one tiny thing I don't get. On 4 and 5, it shows to connect these to the bulb but the thing is, is 4 minus and 5 plus or is 4 plus and 5 minus? I did see a -4 on the ballast but is 4 minus? If anyone would help, I would appreciate it

The link to ballast:

https://www.ebay.co.uk/itm/146262765190

There is no "plus" nor "minus", you may connect it either way.

SOX (and practically all general lighting lamps, except LEDs) are AC lamps, so there is no "+" or "-".
On some lamps (mainly those requiring HV ignition) you differentiate between a "hot" vs "cold" end - "hot" means there is significant potential vs ground, "cold" means the potential is either zero or minimum (with electronic HF ballasts this usually refers to the potential of the high frequency, the "hot" then needs to be wired by as short wires as possible, the required distance to e.g. the other lamp end then should be reached by the "cold" terminal wires, with HV pulse ignition the "hot" is the lead where the HV pulse is present, so on the E-style sockets it should be connected to the center contact)

Practically all SOX are symmetrical by design, so the lamps do not care either.

So basically you can connect Plus to 4 and minus to 5 or the other way round

Yes, exactly...

I'm just puzzled where you get the "plus" and "minus" from, when there is really no electrical, nor mechanical difference between them both on the lamp and socket... :-)

SOX lamps use B22 base. This means the pins can be switched around just by turning the lamp 180°. Polarity does not matter here.

Yes B22 caps so no difference for polarity here anyway, but if you are really concerned then on these, terminal 4 is linked to neutral and terminal 5 is the line out from the ballast coil (with the ignitor being wired internally in parallel with 4 & 5).