I just love the complexities of token saving, and the chaos it brings.
I find it easier to think of the 10 tokens you are given at the start of each race as "appearance money". As long as you cross the start line you get 10 tokens that you can spend on that race, so you can never be without tokens.
It is indeed possible to run the Lotus for 4 consecutive races. You simply need to have saved 40 tokens in your account before you use the Lotus for the first time.
So, for the first time in the Lotus you pay 20 tokens, leaving you with 20 in your account. But you also receive 10 tokens as you cross the start line for that race and that boosts your account to 30 tokens to carry forward .
For the next race you spend 20 tokens for the Lotus again, leaving you with 10 tokens in your account. But you receive another 10 tokens for starting this race boosting your account to 20 tokens to carry forward.
You choose the Lotus for a 3rd race and pay 20 tokens, leaving you with zero in your account, but you get 10 for crossing the start line and now have 10 tokens to carry forward.
If you choose the Lotus for a 4th race you will use the 10 tokens that you saved + the 10 for crossing the start line in the 4th race leaving you with Zero.
But because you will always get 10 tokens just for crossing the start line you can always afford any car up to and including the Brabham.
Of course this means that you will need to drive the BRM for a few races to save your tokens, or the mighty Honda for even more races, if you want to use the Lotus for multi races.
Or you can be safe and boring and use the Brabham for every round, knowing that it is fast enough to trouble the "elite " cars on all but the most high speed of circuits and is more likely to get you to the finish anyway. Plus it is certainly a faster and easier to drive car than those cheaper options.
You have the tokens, you make the choice.