From what I've read here I conclude that Ian, Adam and Roy have the right idea. The Mk4 could certainly benefit from a battery with a higher rating. This has been very well summed up by Ambermarine. When I bought my 'new' Mk4 last August it came with a Bosch battery as shown here:
At first I felt rather smug because I thought that my Bosch battery was the one that Eurocarparts lists for the Mk 4, i.e. the 'big' Bosch that Adam has in his car. Closer examination tells me that my Bosch battery is a standard 70 Ah battery. My point is that a more desirable battery would be this one:
http://www.eurocarparts.com/ecp/c/Lexus_LS_4.0_1998/p/car-parts/car-electrics-and-car-lighting/electrical/car-battery/?444773357&1&990657e924e55c2b108f138aeffff6712c473b81&000020
This one is a 95 Ah battery with 830 cca, no doubt a better option particularly if one's Mk 4 is used only occasionally.
Externally the only difference between the two batteries is their length, 261 mm vs 306 mm (slightly less than 2 inches). Adam appears to have been able to shoehorn his into his Mk 4. My Bosch battery was fitted in December 2010 and it should be coming up for replacement soon. I'll be giving the 'big' Bosch battery some serious thought.