Dividing the Map Grid by half

I am about to run a campaign with creatures that may be smaller than medium.  I am looking for a way to divide the 5ft grid into 4 equal squares to indicate that more than one creature may fit there simultaneously. I've various tricks to get it to reduce increase the number of grid squares but I end up with all kinds of wacky results, including doubling the size of a single square or it looks close but when you use the ruler it comes out with weird numbers that don't quite add up to what it should. What numbers should I enter into the grid sizing to do this?

You want to change "cell width" to 0.5.  Note: do not set up tokens on a map that have a cell width of other 1.0. It will make them come in oddly sized. 

What do you mean do not set up tokens?  If I don't use tokens, kinda makes the map pointless?

When you set up a token set it up on a map that has a cell width of 1.0. Then use it as desired on any other map.  If you set up the token on a map that has a cell width of 0.5 the token will come in at half size when you drag it from the Journal tab. Example:  You create your 0.5 cell width map.  You set up a token on that map (link it to the character, update the token to the character, etc). Next, you drag the character from the journal tab to the map.  Surprise! It comes in at 0.25 cell width, not 0.5. Frustration ensues.  Set it up on a 1.0 cell width map. Then it will come in correctly sized on the 0.5 cell width map when you drag it from the journal tab. 

I kinda expected that, I thought weird meant buggy.  It is the regrettable consequence of needing a grid with more grid control than "medium squares."   I also noticed that when I set the cell width to 0.5 I also had to set the grid cell distance to 2.5  Otherwise, measurements were no longer accurate. Anyways, tyvm for the assistance!  It was very much appreciated.

The measurements being inaccurate would be expected since you are changing the definition of a grid square from 5 feet to 2.5 feet. You'd need to change the measurement distance per grid square.  Usually most folks just hold the Alt key to fit 4 tiny creatures into one 5' square. It avoids all the weirdness (both Roll20 and visually) of the 0.5 cell width.