The four groups acquired more than 10% of shares bought in a £12bn rights issue conducted by the Scottish bank just months before its collapse in 2008.
It was revealed at the High Court of Justice in London yesterday that part-nationalised RBS estimates law firm Herbert Smith will rack up a £42 million bill defending it and former directors Fred Goodwin, Sir Tom McKillop, Johnny Cameron and Guy Whittaker against as many as four separate compensation claims that allege they misled shareholders about the strength of the bank at the time of the fundraising.
The hearing was attended by lawyers from Bird & Bird representing Royal Bank of Scotland Shareholder Action Group which claims backing from 12,500 private investors and 100 institutions and a group of 53 institutions represented by Stewarts Law. Both have already launched actions.
US firm Quinn Emanuel Urquhart and Sullivan, which has been hired by the four investment giants, and Leon Kaye, which fronts the 8000-strong RBS Rights Issue Action Group, also appeared although neither has submitted a formal compensation claim.
Robert Snowden QC, for Quinn Emanuel's group, said: "They are institutions with responsibilities to many millions of stakeholders. They haven't as yet decided whether to commence proceedings."
A key point of dispute between the groups is the allocation of costs, which as things stand could land largely on smaller investors.
Michael Lazarus, representing Leon Kaye's RBS Rights Issue Action Group said that 55% of its members had claims of less than £5000 but were "under the threat of bearing an unequal share of Herbert Smith's £42m cost bill".
Mr Lazarus described his group's members as investors who are "very small in many cases and unsophisticated in many cases to the extent of not having email".
It was agreed a hearing will be held on November 12 to determine how RBS's costs would be split among the groups if the bank won.
Mr Justice Hildyard determined that the cases against RBS will be combined in a group litigation order to make them easier for the court to manage.
He said he remains inclined to enforce a "put-up-or-shut-up" date of April 15 for new claims.
After what Andrew Onslow QC for Stewarts Law described as a "turf war" between the groups as to whose case particulars should form the basis for proceedings, Mr Justice Hildyard ruled that Bird & Bird's submission should take the lead.
But he dismissed an RBS call for a single firm of solicitors to lead the claimants' case and said each group will continue to instruct its own set of lawyers, although he warned he would clamp down on those who "try to gild the lily" on costs.
RBS will set out its defence at a hearing at the end of November.
There remain divisions between the groups on whether to proceed with test cases and how to calculate investors' losses.
A spokesman for the RBoS Shareholder Action Group said: "In a few months we will finally hear how RBS intends to defend the information put in the 2008 rights issue prospectus which, we believe, was deliberately misleading and caused thousands of individuals and institutions to lose billions of pounds."
An RBS spokeswoman said: "The group considers that it has substantial and credible legal and factual defences to the claims and will defend itself vigorously."