4mm is really too wide a gap, 2mm is all you need & if it’s possible to jack the bath up, I would advise you do so to reduce it. At 4mm, your asking a lot even for a quality silicone sealer around a bath; the seal will also be quiet large & it won’t look so good either. If you can’t reduce the gap, use a quality low modulus neutral cure silicone to give better odds of it surviving but you probably won’t get it at any DIY shed, they usually only stock, much cheaper high modulus, acetoxy cure - smells of vinegar.
As Andy, grout first, thoroughly clean the bath/tile surfaces with an alcohol based spirit & fill the bath as high as possible before applying silicone. I silicone in two stages; first run a good bead between the underside of the tile & the top of the bath using an uncut nozzle forcing silicone into the gap (I allow 2mm). Clean off any excess, cut the nozzle to the required bead size & immediately run a second silicone bead between the front face of the tile & the bath. Pull/angle the gun away from the bead, don’t push it into it; finish off with a wet finger (not spit). Leave the bath full overnight to give the silicone a chance to fully cure.
