https://www.researchgate.net/publication/281775607_Solubility_of_Oxalic_Acid

it may guide to some extent

Article Solubility of Oxalic Acid

Dear Sir. Concerning your issue about the possibility of preparation of a 4M oxalic acid solution. The solubility of Oxalic acid in aqueous media increase with increase temperature while decrease with decrease temperature at constant pressure and electrolytes. The solubility of the oxalic acid in water is 220,000 mg/L at 25 deg C (Yalkowsky SH et al; Arizona Data Base of Water Solubility (1989)). A solution of oxalic acid in water at temperatures below 70° practically all the molecules present are bound to (at least) two molecules of water, but that at higher temperatures an increasing dissociation into anhydrous acid and water takes place The following below links may help you in your analysis:

https://pubchem.ncbi.nlm.nih.gov/compound/oxalic_acid#section=Melting-Point

http://www.flinnsci.com/media/396156/labsolutionprep.pdf

http://onlinelibrary.wiley.com/doi/10.1002/recl.19500690102/abstract

https://www.researchgate.net/publication/281775607_Solubility_of_Oxalic_Acid

Thanks

Article Solubility of Oxalic Acid

 Dear Lee Ifhar, Hi,

• According to a calculation, to prepare 100 mL.4M Oxalic acid, you need 36 g of it. If you see the paper sent to You,thankfully, by Mushtaq Ahmad, you will find that the solubility of this acid at 30 deg  C is only about 6 g per 100 mL water. This means, even for 2 M of the acid needing 18 g of the acid added to 100 mL water, you have to heat the solution up to over 70 deg C according to the same table. Unfortunately 4 M preparation of Oxalic acid seems not to be possible. Thank You and Good Luck