You probably wont want to go with a rosco color, R360 is closish, but L202 (Lee, http://www.leefilters.com/) is your best bet with a source 4. This will get rid of the amberness of the source and bump the temp a bit, but as said before its all relative to what it is around.
And some peoples white is bluish. Plus it also depends what else is on stage < if this is theatre> The problem with White,white,white,white,whight,wygte,wyght,Light is that I'll make everything Glow. The least amount of a primary is REALLY, going to pop.
Do a little internet research on the term "color temperature". I don't have time to find a good site for you right now but I'm sure someone will have something to help. Once you know what color temperature you are looking for then there are lots of things to do to achieve that.
Maybe I'm reading this wrong, but I'm reminded of the time I programmed a show in two hours, two hours before the house opened. I accidentally had the intensity up like 15 percent above where it should be. The result was every scene looked, for lack of better words, searing. (Painfully bright white light.)
I'm thinking perhaps this user isn't looking to define the color temperature, of white, but create that painfully bright white. I only have limited experience, so take all my advice with a truck of salt, but I'd try (coming from an inventory primarily composed of 360Qs) some Source Fours, run at full. Also, as mentioned, look into bumping up the color temperature. Removing some of the amber with a bluer gel seems like a good idea too.
You are right Charc, that is one approach but there are a lot of approaches. You really have to define what you mean by white and how you want it used. You can create "whiter" light by boosting lumen output, altering color blends, using instruments with higher color temperature lamps, and using filters to change color temperature... it all depends on what you are trying to do.
For my purposes, I will typically use the Rosco suite of CTB gells. CTB or CTO are used primarily in film or television to make the appearance of the light "warmer" or "cooler" as it pertains to color temperature. A standard S4 lamp (tungesten) will produce a color temp of 3200K which, in the grand scheme of things, is on the amber or "warm" side of the specturm.
However, if you want to make it "cooler" or less amber and more blue (which can appear more like true WHITE depending on the desired effect) you can use one of the varying degrees of CTB to increase the color temperature.
The advantages to using CTB or CTO in a fixture is that in only adjusts color temperature and not the hue like many other gels do. In theater this is less crucial and R60 is a perfect supliment to get a color temp close to 5200K which will match the color temp of most short-arc fixtures (moving lights, HMI fixtures...Many people refer to the color temperature of MLs and HMI fixtures to be true WHITE--that is in the eye of the beholder). However, when you need to color balance for film or tv, it is crucial to achieve the exact color temperature.
Both CTB and CTO can be purchased in 1/8, 1/4, 1/2, and FULL incriments, with varying degrees of impact. I believe FULL CTB will take 3200k up to 5300K or WHITE as it pertains to MLs-but don't quote me on that. It will say in the gelbook.
A combination of luminous output and color temperature - either color corrected or by way of output than is the normal solution. This when added to dimmer levels and color temperature & intensity of other fixtures about this "white light." A candle will perhaps not go too white when compared to a black setting but a fluorescent lamp will be white when compared to this. On the other hand a xenon follow spot will be intensely bright in comparison to that fluorescent. This at least by way of color temperature.
Put a Fresnel with top hat focused directly down with a similar Fresnel washing the rest of the stage. Turn the Fresnels up to full and spotted and tophatted downlight to 50% and it won't be bright in comparison. Reverse this and even if the same color temperature range, that luminous focused output will be more white due to amber shift and luminous output than the surroundings this without any gel added.
This is a combination of focus of light by way of turning the Fresnel to spot position & lessening the others in wash mode by way of intensity so they have amber shift to lower both output and color temperature.
Note there is even color corrected PAR 64 lamps on the market that would go white light without needing gel or a moving light or follow spot. The possibly discontinued GE/Thorn FGM & FGN series of PAR 64 lamp is a very "daylight deluxe" 5,200̊K in color temperature, now thats' cool or white.
Just in case the whole "White" issue was not confusing enough, the human eye has a non-liner color shift when intensity changes! Example: If you use a specific color temperature and shine it through a grey scale chart, the darker elements will appear to shift blue, even though all the elements of the chart are neutral grey! (If you ever have done camera alignments, you probably already know this.) Best real life example is viewing a MH streetlight close up (very white) as compared to viewing it's output reflected at a distance. (rather blue) Although the eye is most sensitive to green, all three colors the eye sees (RGB) track differently with intensity producing a "gamma" curve graph with the scoop of the gamma curve at three different places on the intensity graph. This is one of the reasons that a higher color temperature source appears brighter even if the light level is the same.